We don't need to rebuild objfiles.txt every time an object file changes.

So only rebuild when a makefile changes (which presumably defines the
file list somewhere), and only touch the file if an object changed. The
touch is necessary so the parent make knows something changed and
ultimately rebuilds postgres.

src/backend/common.mk

 #
 # Common make rules for backend
 #
-# $PostgreSQL: pgsql/src/backend/common.mk,v 1.2 2008/02/25 17:55:42 petere Exp $
+# $PostgreSQL: pgsql/src/backend/common.mk,v 1.3 2008/02/26 07:20:38 petere Exp $
 #
 
 # When including this file, set OBJS to the object files created in
@@ -26,9 +26,12 @@ endif
 SUBSYS.o: $(SUBDIROBJS) $(OBJS)
 	$(LD) $(LDREL) $(LDOUT) $@ $^
 
-objfiles.txt: $(SUBDIROBJS) $(OBJS)
+objfiles.txt:: $(MAKEFILE_LIST)
 	( $(if $(SUBDIROBJS),cat $(SUBDIROBJS); )echo $(addprefix $(subdir)/,$(OBJS)) ) >$@
 
+objfiles.txt:: $(SUBDIROBJS) $(OBJS)
+	touch $@
+
 # make function to expand objfiles.txt contents
 expand_subsys = $(foreach file,$(filter %/objfiles.txt,$(1)),$(patsubst ../../src/backend/%,%,$(addprefix $(top_builddir)/,$(shell cat $(file))))) $(filter-out %/objfiles.txt,$(1))