make darwin port a little more cross & cross native build friendly.

make darwin port a little more cross & cross native build friendly.

	*config/darwin-driver.c (darwin_find_version_from_kernel): New routine
	cut from ... (darwin_default_min_version): Amended to provide defaults
	for the cross directory case.
	(darwin_driver_init): call darwin_default_min_version unconditionally.
	* config/darwin.h (DEF_MIN_OSX_VERSION): New.
	* config/darwin9.h: Likewise.
	* config/darwin10.h: Likewise.
	* config/rs6000/darwin7.h: Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@178679 138bc75d-0d04-0410-961f-82ee72b054a4