Makefile.in.in: Make dependencies of a PO file and 'dist' customizable
* Makevars (POFILESDEPS): New variable.
(DISTFILESDEPS): New variable.
* Makefile.in.in ($(POFILES)): Depend on $(POFILESDEPS) instead of
$(srcdir)/$(DOMAIN).pot.
(dist distdir): Run $(DISTFILEDEPS) instead of 'update-po'.
