darwin.h (SUBTARGET_OPTIONS): Add -mmacosx= option.

darwin.h (SUBTARGET_OPTIONS): Add -mmacosx= option.

	* config/darwin.h (SUBTARGET_OPTIONS): Add -mmacosx= option.
	(macosx_version_min_required): Declare global variable.
	* config/darwin-protos.h (darwin_parse_macosx_version_name): Declare.
	* config/darwin.c (macosx_version_min_required): Define variable.
	(darwin_macosx_vers_tbl): Define. Table mapping text strings of
	OS versions to numerical codes.
	(darwin_parse_macosx_version_name): Define.  Parse -mmacosx=,
	set macosx_version_min_required appropriately.
	* config/i386/darwin.h (SUBTARGET_OVERRIDE_OPTIONS): Define.
	Invoke darwin_parse_macosx_version_name.
	* config/rs6000/darwin.h (SUBTARGET_OVERRIDE_OPTIONS): Add
	darwin_parse_macosx_version_name to Darwin option handling.
	* doc/invoke.texi (-mmacosx=): Document.

From-SVN: r78329