In r15-3714-gd3a7302ec5985a I added -Wsystem-headers to the libstdc++ build
flags to help catch problems in the library.  This patch takes a different
approach, of disabling the #pragma system_header unless _GLIBCXX_SYSHDR is
defined.  As a result, the testsuites will treat them as non-system-headers
to get better warning coverage during regression testing of both gcc and
libstdc++, not just when building the library.

My rationale for the #ifdef instead of just removing the #pragma is the
three G++ tests that want to test libstdc++ system header behavior, so we
need a way to select it.

This doesn't affect installed libraries, as they get their
system-header status from the lookup path.  But testsuite_flags
--build-includes gives -I directives rather than -isystem.

This patch doesn't change the headers in config/ because I'm not compiling
with most of them, so won't see any warnings that need fixing.  Adjusting
them could happen later, or we can not bother.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (WARN_FLAGS): Remove -Wsystem-headers.
	* configure: Regenerate.
	* include/bits/algorithmfwd.h: #ifdef out #pragma GCC system_header.
	* include/bits/atomic_base.h
	* include/bits/atomic_futex.h
	* include/bits/atomic_timed_wait.h
	* include/bits/atomic_wait.h
	* include/bits/basic_ios.h
	* include/bits/basic_string.h
	* include/bits/boost_concept_check.h
	* include/bits/char_traits.h
	* include/bits/charconv.h
	* include/bits/chrono.h
	* include/bits/chrono_io.h
	* include/bits/codecvt.h
	* include/bits/concept_check.h
	* include/bits/cpp_type_traits.h
	* include/bits/elements_of.h
	* include/bits/enable_special_members.h
	* include/bits/erase_if.h
	* include/bits/forward_list.h
	* include/bits/functional_hash.h
	* include/bits/gslice.h
	* include/bits/gslice_array.h
	* include/bits/hashtable.h
	* include/bits/indirect_array.h
	* include/bits/invoke.h
	* include/bits/ios_base.h
	* include/bits/iterator_concepts.h
	* include/bits/locale_classes.h
	* include/bits/locale_facets.h
	* include/bits/locale_facets_nonio.h
	* include/bits/localefwd.h
	* include/bits/mask_array.h
	* include/bits/max_size_type.h
	* include/bits/memory_resource.h
	* include/bits/memoryfwd.h
	* include/bits/move_only_function.h
	* include/bits/node_handle.h
	* include/bits/ostream_insert.h
	* include/bits/out_ptr.h
	* include/bits/parse_numbers.h
	* include/bits/postypes.h
	* include/bits/quoted_string.h
	* include/bits/range_access.h
	* include/bits/ranges_base.h
	* include/bits/refwrap.h
	* include/bits/sat_arith.h
	* include/bits/semaphore_base.h
	* include/bits/slice_array.h
	* include/bits/std_abs.h
	* include/bits/std_function.h
	* include/bits/std_mutex.h
	* include/bits/std_thread.h
	* include/bits/stl_iterator_base_funcs.h
	* include/bits/stl_iterator_base_types.h
	* include/bits/stl_tree.h
	* include/bits/stream_iterator.h
	* include/bits/streambuf_iterator.h
	* include/bits/stringfwd.h
	* include/bits/this_thread_sleep.h
	* include/bits/unique_lock.h
	* include/bits/uses_allocator_args.h
	* include/bits/utility.h
	* include/bits/valarray_after.h
	* include/bits/valarray_array.h
	* include/bits/valarray_before.h
	* include/bits/version.h
	* include/c_compatibility/fenv.h
	* include/c_compatibility/inttypes.h
	* include/c_compatibility/stdint.h
	* include/decimal/decimal.h
	* include/experimental/bits/net.h
	* include/experimental/bits/shared_ptr.h
	* include/ext/aligned_buffer.h
	* include/ext/alloc_traits.h
	* include/ext/atomicity.h
	* include/ext/concurrence.h
	* include/ext/numeric_traits.h
	* include/ext/pod_char_traits.h
	* include/ext/pointer.h
	* include/ext/stdio_filebuf.h
	* include/ext/stdio_sync_filebuf.h
	* include/ext/string_conversions.h
	* include/ext/type_traits.h
	* include/ext/vstring.h
	* include/ext/vstring_fwd.h
	* include/ext/vstring_util.h
	* include/parallel/algorithmfwd.h
	* include/parallel/numericfwd.h
	* include/tr1/functional_hash.h
	* include/tr1/hashtable.h
	* include/tr1/random.h
	* libsupc++/exception.h
	* libsupc++/hash_bytes.h
	* include/bits/basic_ios.tcc
	* include/bits/basic_string.tcc
	* include/bits/fstream.tcc
	* include/bits/istream.tcc
	* include/bits/locale_classes.tcc
	* include/bits/locale_facets.tcc
	* include/bits/locale_facets_nonio.tcc
	* include/bits/ostream.tcc
	* include/bits/sstream.tcc
	* include/bits/streambuf.tcc
	* include/bits/string_view.tcc
	* include/bits/version.tpl
	* include/experimental/bits/string_view.tcc
	* include/ext/pb_ds/detail/resize_policy/hash_prime_size_policy_imp.hpp
	* include/ext/random.tcc
	* include/ext/vstring.tcc
	* include/tr2/bool_set.tcc
	* include/tr2/dynamic_bitset.tcc
	* include/bits/c++config
	* include/c/cassert
	* include/c/cctype
	* include/c/cerrno
	* include/c/cfloat
	* include/c/ciso646
	* include/c/climits
	* include/c/clocale
	* include/c/cmath
	* include/c/csetjmp
	* include/c/csignal
	* include/c/cstdarg
	* include/c/cstddef
	* include/c/cstdio
	* include/c/cstdlib
	* include/c/cstring
	* include/c/ctime
	* include/c/cuchar
	* include/c/cwchar
	* include/c/cwctype
	* include/c_global/cassert
	* include/c_global/ccomplex
	* include/c_global/cctype
	* include/c_global/cerrno
	* include/c_global/cfenv
	* include/c_global/cfloat
	* include/c_global/cinttypes
	* include/c_global/ciso646
	* include/c_global/climits
	* include/c_global/clocale
	* include/c_global/cmath
	* include/c_global/csetjmp
	* include/c_global/csignal
	* include/c_global/cstdalign
	* include/c_global/cstdarg
	* include/c_global/cstdbool
	* include/c_global/cstddef
	* include/c_global/cstdint
	* include/c_global/cstdio
	* include/c_global/cstdlib
	* include/c_global/cstring
	* include/c_global/ctgmath
	* include/c_global/ctime
	* include/c_global/cuchar
	* include/c_global/cwchar
	* include/c_global/cwctype
	* include/c_std/cassert
	* include/c_std/cctype
	* include/c_std/cerrno
	* include/c_std/cfloat
	* include/c_std/ciso646
	* include/c_std/climits
	* include/c_std/clocale
	* include/c_std/cmath
	* include/c_std/csetjmp
	* include/c_std/csignal
	* include/c_std/cstdarg
	* include/c_std/cstddef
	* include/c_std/cstdio
	* include/c_std/cstdlib
	* include/c_std/cstring
	* include/c_std/ctime
	* include/c_std/cuchar
	* include/c_std/cwchar
	* include/c_std/cwctype
	* include/debug/array
	* include/debug/bitset
	* include/debug/deque
	* include/debug/forward_list
	* include/debug/list
	* include/debug/map
	* include/debug/set
	* include/debug/string
	* include/debug/unordered_map
	* include/debug/unordered_set
	* include/debug/vector
	* include/decimal/decimal
	* include/experimental/algorithm
	* include/experimental/any
	* include/experimental/array
	* include/experimental/buffer
	* include/experimental/chrono
	* include/experimental/contract
	* include/experimental/deque
	* include/experimental/executor
	* include/experimental/filesystem
	* include/experimental/forward_list
	* include/experimental/functional
	* include/experimental/internet
	* include/experimental/io_context
	* include/experimental/iterator
	* include/experimental/list
	* include/experimental/map
	* include/experimental/memory
	* include/experimental/memory_resource
	* include/experimental/net
	* include/experimental/netfwd
	* include/experimental/numeric
	* include/experimental/propagate_const
	* include/experimental/ratio
	* include/experimental/regex
	* include/experimental/scope
	* include/experimental/set
	* include/experimental/socket
	* include/experimental/string
	* include/experimental/string_view
	* include/experimental/synchronized_value
	* include/experimental/system_error
	* include/experimental/timer
	* include/experimental/tuple
	* include/experimental/type_traits
	* include/experimental/unordered_map
	* include/experimental/unordered_set
	* include/experimental/vector
	* include/ext/algorithm
	* include/ext/cmath
	* include/ext/functional
	* include/ext/iterator
	* include/ext/memory
	* include/ext/numeric
	* include/ext/random
	* include/ext/rb_tree
	* include/ext/rope
	* include/parallel/algorithm
	* include/std/algorithm
	* include/std/any
	* include/std/array
	* include/std/atomic
	* include/std/barrier
	* include/std/bit
	* include/std/bitset
	* include/std/charconv
	* include/std/chrono
	* include/std/codecvt
	* include/std/complex
	* include/std/concepts
	* include/std/condition_variable
	* include/std/coroutine
	* include/std/deque
	* include/std/execution
	* include/std/expected
	* include/std/filesystem
	* include/std/format
	* include/std/forward_list
	* include/std/fstream
	* include/std/functional
	* include/std/future
	* include/std/generator
	* include/std/iomanip
	* include/std/ios
	* include/std/iosfwd
	* include/std/iostream
	* include/std/istream
	* include/std/iterator
	* include/std/latch
	* include/std/limits
	* include/std/list
	* include/std/locale
	* include/std/map
	* include/std/memory
	* include/std/memory_resource
	* include/std/mutex
	* include/std/numbers
	* include/std/numeric
	* include/std/optional
	* include/std/ostream
	* include/std/print
	* include/std/queue
	* include/std/random
	* include/std/ranges
	* include/std/ratio
	* include/std/regex
	* include/std/scoped_allocator
	* include/std/semaphore
	* include/std/set
	* include/std/shared_mutex
	* include/std/span
	* include/std/spanstream
	* include/std/sstream
	* include/std/stack
	* include/std/stacktrace
	* include/std/stdexcept
	* include/std/streambuf
	* include/std/string
	* include/std/string_view
	* include/std/syncstream
	* include/std/system_error
	* include/std/text_encoding
	* include/std/thread
	* include/std/tuple
	* include/std/type_traits
	* include/std/typeindex
	* include/std/unordered_map
	* include/std/unordered_set
	* include/std/utility
	* include/std/valarray
	* include/std/variant
	* include/std/vector
	* include/std/version
	* include/tr1/array
	* include/tr1/cfenv
	* include/tr1/cinttypes
	* include/tr1/cmath
	* include/tr1/complex
	* include/tr1/cstdbool
	* include/tr1/cstdint
	* include/tr1/cstdio
	* include/tr1/cstdlib
	* include/tr1/cwchar
	* include/tr1/cwctype
	* include/tr1/functional
	* include/tr1/memory
	* include/tr1/random
	* include/tr1/regex
	* include/tr1/tuple
	* include/tr1/type_traits
	* include/tr1/unordered_map
	* include/tr1/unordered_set
	* include/tr1/utility
	* include/tr2/bool_set
	* include/tr2/dynamic_bitset
	* include/tr2/type_traits
	* libsupc++/atomic_lockfree_defines.h
	* libsupc++/compare
	* libsupc++/cxxabi.h
	* libsupc++/cxxabi_forced.h
	* libsupc++/cxxabi_init_exception.h
	* libsupc++/exception
	* libsupc++/initializer_list
	* libsupc++/new
	* libsupc++/typeinfo: Likewise.
	* testsuite/20_util/ratio/operations/ops_overflow_neg.cc
	* testsuite/23_containers/array/tuple_interface/get_neg.cc
	* testsuite/23_containers/vector/cons/destructible_debug_neg.cc
	* testsuite/24_iterators/operations/prev_neg.cc
	* testsuite/ext/type_traits/add_unsigned_floating_neg.cc
	* testsuite/ext/type_traits/add_unsigned_integer_neg.cc
	* testsuite/ext/type_traits/remove_unsigned_floating_neg.cc
	* testsuite/ext/type_traits/remove_unsigned_integer_neg.cc: Adjust
	line numbers.

gcc/testsuite/ChangeLog

	* g++.dg/analyzer/fanalyzer-show-events-in-system-headers-default.C
	* g++.dg/analyzer/fanalyzer-show-events-in-system-headers-no.C
	* g++.dg/diagnostic/disable.C: #define _GLIBCXX_SYSHDR.