maint.mk: exceptions through variables instead of hidden files.

This allows variables of the form VC_LIST_EXCEPT_$@ which contain an
extended regular exception matching file names to be excluded from the
syntax check $@.  The old method of files $(srcdir)/.x-$@ containing
exception paterns is still supported for backwards compatibility, but I
personally would discourage them, as they cause a lot of files for things
better maintained in a single file, and as hidden files with unclear file
names might cause unnecessary amounts of confusion.

top/maint.mk

@@ -59,7 +59,7 @@ endif
 VC_LIST_EXCEPT = \
   $(VC_LIST) | sed 's|^$(_dot_escaped_srcdir)/||' \
 	| if test -f $(srcdir)/.x-$@; then grep -vEf $(srcdir)/.x-$@; \
-	  else grep -Ev -e "$${VC_LIST_EXCEPT_DEFAULT-ChangeLog}"; fi \
+	  else grep -Ev -e $(if $(VC_LIST_EXCEPT_$@),'$(VC_LIST_EXCEPT_$@)',"$${VC_LIST_EXCEPT_DEFAULT-ChangeLog}"); fi \
 	| grep -Ev -e '$(VC_LIST_ALWAYS_EXCLUDE_REGEX)' \
 	$(_prepend_srcdir_prefix)