Add @configure_input@ marker to Makefile.global.in

That way, when looking at Makefile.global, we don't get confused by the
comment that claims that this is Makefile.global.in.

src/Makefile.global.in

 # -*-makefile-*-
 # src/Makefile.global.in
+# @configure_input@
 
 #------------------------------------------------------------------------------
 # All PostgreSQL makefiles include this file and use the variables it sets,