chrono.cc: If _GLIBCXX_USE_CLOCK_GETTIME_SYSCALL, include unistd.h and sys/syscall.h.

	* src/c++11/chrono.cc: If _GLIBCXX_USE_CLOCK_GETTIME_SYSCALL,
	include unistd.h and sys/syscall.h.  If _GLIBCXX_COMPATIBILITY_CXX0X,
	don't define system_clock::is_steady, system_clock::now() and
	steady_clock::is_steady.
	(std::chrono::system_clock::now()): If
	_GLIBCXX_USE_CLOCK_GETTIME_SYSCALL, call
	syscall (SYS_clock_gettime, ...) instead of clock_gettime (...).
	(std::chrono::system_clock::now()): Likewise.  Add weak attribute
	if _GLIBCXX_COMPATIBILITY_CXX0X and compatibility-chrono.cc will
	be non-empty.
	* src/Makefile.am (cxx11_sources): Add compatibility-chrono.cc.
	(compatibility-chrono.lo, compatibility-chrono.o): New goals.
	* src/c++11/compatibility-chrono.cc: New file.
	* acinclude.m4 (GLIBCXX_ENABLE_LIBSTDCXX_TIME): On linux*, check for
	syscall (SYS_clock_gettime, CLOCK_MONOTONIC, &tp).
	* testsuite/util/testsuite_abi.cc (check_version): Add
	GLIBCXX_3.4.20 version and make it the latest.
	* config/abi/pre/gnu.ver (_ZNSt6chrono12steady_clock3nowEv): Export
	also @@GLIBCXX_3.4.19.  Move all symbols so far added for GCC 4.9 to
	@@GLIBCXX_3.4.20 instead.
	* config/abi/post/i386-linux-gnu/baseline_symbols.txt: Regenerated.
	* config/abi/post/s390x-linux-gnu/baseline_symbols.txt: Regenerated.
	* config/abi/post/powerpc64-linux-gnu/baseline_symbols.txt:
	Regenerated.
	* config/abi/post/powerpc64-linux-gnu/32/baseline_symbols.txt:
	Regenerated.
	* config/abi/post/x86_64-linux-gnu/baseline_symbols.txt: Regenerated.
	* config/abi/post/x86_64-linux-gnu/32/baseline_symbols.txt:
	Regenerated.
	* config/abi/post/powerpc-linux-gnu/baseline_symbols.txt: Regenerated.
	* config/abi/post/s390-linux-gnu/baseline_symbols.txt: Regenerated.
	* config/abi/post/i486-linux-gnu/baseline_symbols.txt: Regenerated.
	* config/abi/post/solaris2.10/baseline_symbols.txt: Regenerated.
	* config/abi/post/solaris2.10/amd64/baseline_symbols.txt: Regenerated.
	* config/abi/post/solaris2.10/sparcv9/baseline_symbols.txt:
	Regenerated.
	* config/abi/post/solaris2.9/baseline_symbols.txt: Regenerated.
	* config/abi/post/solaris2.9/sparcv9/baseline_symbols.txt: Regenerated.
	* config.h.in: Regenerated.
	* src/Makefile.in: Regenerated.
	* configure: Regenerated.

From-SVN: r199307