Recent Darwin versions place contraints on the use of run paths
specified in environment variables.  This breaks some assumptions
in the GCC build.

This change allows the user to configure a Darwin build to use
'@rpath/libraryname.dylib' in library names and then to add an
embedded runpath to executables (and libraries with dependents).

The embedded runpath is added by default unless the user adds
'-nodefaultrpaths' to the link line.

For an installed compiler, it means that any executable built with
that compiler will reference the runtimes installed with the
compiler (equivalent to hard-coding the library path into the name
of the library).

During build-time configurations  any "-B" entries will be added to
the runpath thus the newly-built libraries will be found by exes.

Since the install name is set in libtool, that decision needs to be
available here (but might also cause dependent ones in Makefiles,
so we need to export a conditional).

This facility is not available for Darwin 8 or earlier, however the
existing environment variable runpath does work there.

We default this on for systems where the external DYLD_LIBRARY_PATH
does not work and off for Darwin 8 or earlier.  For systems that can
use either method, if the value is unset, we use the default (which
is currently DYLD_LIBRARY_PATH).

ChangeLog:

	* configure: Regenerate.
	* configure.ac: Do not add default runpaths to GCC exes
	when we are building -static-libstdc++/-static-libgcc (the
	default).
	* libtool.m4: Add 'enable-darwin-at-runpath'.  Act  on the
	enable flag to alter Darwin libraries to use @rpath names.

gcc/ChangeLog:

	* aclocal.m4: Regenerate.
	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.
	* config/darwin.h: Handle Darwin rpaths.
	* config/darwin.opt: Handle Darwin rpaths.
	* Makefile.in:  Handle Darwin rpaths.

gcc/ada/ChangeLog:

	* gcc-interface/Makefile.in: Handle Darwin rpaths.

gcc/jit/ChangeLog:
	* Make-lang.in: Handle Darwin rpaths.

libatomic/ChangeLog:

	* Makefile.am: Handle Darwin rpaths.
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.

libbacktrace/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.

libcc1/ChangeLog:

	* configure: Regenerate.

libffi/ChangeLog:

	* Makefile.am: Handle Darwin rpaths.
	* Makefile.in: Regenerate.
	* configure: Regenerate.

libgcc/ChangeLog:

	* config/t-slibgcc-darwin: Generate libgcc_s
	with an @rpath name.
	* config.host: Handle Darwin rpaths.

libgfortran/ChangeLog:

	* Makefile.am: Handle Darwin rpaths.
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths

libgm2/ChangeLog:

	* Makefile.am: Handle Darwin rpaths.
	* Makefile.in: Regenerate.
	* aclocal.m4: Regenerate.
	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.
	* libm2cor/Makefile.am: Handle Darwin rpaths.
	* libm2cor/Makefile.in: Regenerate.
	* libm2iso/Makefile.am: Handle Darwin rpaths.
	* libm2iso/Makefile.in: Regenerate.
	* libm2log/Makefile.am: Handle Darwin rpaths.
	* libm2log/Makefile.in: Regenerate.
	* libm2min/Makefile.am: Handle Darwin rpaths.
	* libm2min/Makefile.in: Regenerate.
	* libm2pim/Makefile.am: Handle Darwin rpaths.
	* libm2pim/Makefile.in: Regenerate.

libgomp/ChangeLog:

	* Makefile.am: Handle Darwin rpaths.
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths

libitm/ChangeLog:

	* Makefile.am: Handle Darwin rpaths.
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.

libobjc/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.

libphobos/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.
	* libdruntime/Makefile.am: Handle Darwin rpaths.
	* libdruntime/Makefile.in: Regenerate.
	* src/Makefile.am: Handle Darwin rpaths.
	* src/Makefile.in: Regenerate.

libquadmath/ChangeLog:

	* Makefile.am: Handle Darwin rpaths.
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.

libsanitizer/ChangeLog:

	* asan/Makefile.am: Handle Darwin rpaths.
	* asan/Makefile.in: Regenerate.
	* configure: Regenerate.
	* hwasan/Makefile.am: Handle Darwin rpaths.
	* hwasan/Makefile.in: Regenerate.
	* lsan/Makefile.am: Handle Darwin rpaths.
	* lsan/Makefile.in: Regenerate.
	* tsan/Makefile.am: Handle Darwin rpaths.
	* tsan/Makefile.in: Regenerate.
	* ubsan/Makefile.am: Handle Darwin rpaths.
	* ubsan/Makefile.in: Regenerate.

libssp/ChangeLog:

	* Makefile.am: Handle Darwin rpaths.
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.

libstdc++-v3/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.
	* src/Makefile.am: Handle Darwin rpaths.
	* src/Makefile.in: Regenerate.

libvtv/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.

lto-plugin/ChangeLog:
	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.

zlib/ChangeLog:
	* configure: Regenerate.
	* configure.ac: Handle Darwin rpaths.