* build-all.mk: If host not specified, use config.guess. Pass TAG

	* build-all.mk: If host not specified, use config.guess.  Pass TAG
	to test-build.mk as RELEASE_TAG.
	* test-build.mk (configargs): New variable containing arguments to
	pass to configure.  Set to -with-gnu-as on mips-dec-ultrix.
	(FLAGS_TO_PASS): Pass down RELEASE_TAG.

	* config.guess: Use /bin/uname when checking -X argument on SCO,
	to avoid invoking GNU uname which doesn't understand -X.