Only support -gstabs on Mac OS X if assember supports it (PR target/67973)

Only support -gstabs on Mac OS X if assember supports it (PR target/67973)

	gcc:
	PR target/67973
	* configure.ac (gcc_cv_as_stabs_directive): New test.
	* configure: Regenerate.
	* config.in: Regenerate.
	* config/darwin.h (DBX_DEBUGGING_INFO): Wrap in
	HAVE_AS_STABS_DIRECTIVE.
	(PREFERRED_DEBUGGING_TYPE): Likewise.
	* config/i386/darwin.h (PREFERRED_DEBUGGING_TYPE): Only include
	DBX_DEBUG if HAVE_AS_STABS_DIRECTIVE.

	* doc/sourcebuild.texi (Effective-Target Keywords, Environment
	attributes): Document stabs.

	gcc/testsuite:
	PR target/67973
	* lib/target-supports.exp (check_effective_target_stabs): New
	proc.
	* g++.dg/cpp0x/alias-decl-debug-0.C: Restrict to stabs targets.
	* g++.dg/other/PR23205.C: Likewise.
	* g++.dg/other/pr23205-2.C: Likewise.
	* gcc.dg/20040813-1.c: Likewise.
	* gcc.dg/darwin-20040809-2.c: Likewise.
	* objc.dg/stabs-1.m: Likewise.

From-SVN: r231747