Add an implicit rule %.c -> %.i for running the C preprocessor.

I occasionally use this for debugging, and it seems wasteful to have to
reinvent this all the time.

src/Makefile.global.in

 # -*-makefile-*-
-# $PostgreSQL: pgsql/src/Makefile.global.in,v 1.252 2009/01/22 22:27:13 petere Exp $
+# $PostgreSQL: pgsql/src/Makefile.global.in,v 1.253 2009/02/19 10:32:30 petere Exp $
 
 #------------------------------------------------------------------------------
 # All PostgreSQL makefiles include this file and use the variables it sets,
@@ -468,6 +468,9 @@ TAS         = @TAS@
 #
 # Global targets and rules
 
+%.i: %.c
+	$(CPP) $(CPPFLAGS) -o $@ $<
+
 %.gz: %
 	$(GZIP) -f --best $<