automake: Move the master Mesa makefile to Makefile.old.

This will let me incrementally move stuff to automake without
converting libmesa.a all at once.
diff --git a/configure.ac b/configure.ac
index e41a73d..b2f561d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2171,6 +2171,7 @@
 		src/mapi/shared-glapi/Makefile
 		src/mapi/glapi/tests/Makefile
 		src/gtest/Makefile
+		src/mesa/Makefile
 		src/mesa/libdricore/Makefile
 		src/mesa/main/tests/Makefile
 		src/mesa/x86/Makefile