Darwin - Tidy mmacosx-version-min code and specs.

Darwin - Tidy mmacosx-version-min code and specs.

gcc/

	* config/darwin-driver.c (darwin_default_min_version): Refactor code.
	(darwin_driver_init): Note a version-min when provided on the c/l.
	* config/darwin.h (%darwin_minversion): Remove.
	* config/i386/darwin.h: Likewise.
	* config/rs6000/darwin.h: Likewise.
	* config/darwin.opt (mmacosx-version-min=): Use the configured default, rather than
	an arbitrary constant.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@228944 138bc75d-0d04-0410-961f-82ee72b054a4