Change some dependency rules to make them work with VMS.

The VMS make utilities always need an action if they determine that a target is
older than a denpendency element.

Besides, rules like

some/header.h : some/source.c

some/source.c : some/file.dump
	$(PMC2CC) some/source.pmc

don't reflect the building process, as both the C and header file are generated
by the $(PMC2CC) call.