Makefile.in: clean up match.pd-related dependencies

Clean up confusing changes from the recent refactoring for
parallel match.pd build.

gimple-match-head.o is not built. Remove related flags adjustment.

Autogenerated gimple-match-N.o files do not depend on
gimple-match-exports.cc.

{gimple,generic)-match-auto.h only depend on the prerequisites of the
corresponding s-{gimple,generic}-match stamp file, not any .cc file.

gcc/ChangeLog:

	* Makefile.in: (gimple-match-head.o-warn): Remove.
	(GIMPLE_MATCH_PD_SEQ_SRC): Do not depend on
	gimple-match-exports.cc.
	(gimple-match-auto.h): Only depend on s-gimple-match.
	(generic-match-auto.h): Likewise.