If we're gonna have people running make from the top directory now,

we'd better have a Makefile here to prevent the mistake of using
vannilla make instead of gmake.  (But let's leave src/Makefile where
it is, too.)

Makefile

+# The Postgres make files exploit features of GNU make that other makes
+# do not have.  Because it is a common mistake for users to try to build
+# Postgres with a different make, we have this make file that does nothing
+# but tell the user to use GNU make.
+
+# If the user were using GNU make now, this file would not get used because
+# GNU make uses a make file named "GNUmakefile" in preference to "Makefile"
+# if it exists.  Postgres is shipped with a "GNUmakefile".
+
+all install clean dep depend distclean:
+	@echo "You must use GNU make to use Postgres.  It may be installed"
+	@echo "on your system with the name 'gmake'."
+	@echo
+	@echo "NOTE:  If you are sure that you are using GNU make and you are"
+	@echo "       still getting this message, you may simply need to run"
+	@echo "       the configure program."