autoconf: Add gcc search patch to MKDEP_OPTIONS to eliminate warnings.

Also, use -include to avoid error message when make initially fails to
include the non-existent depend file.
diff --git a/src/glx/x11/Makefile b/src/glx/x11/Makefile
index 00a568c..3366f00 100644
--- a/src/glx/x11/Makefile
+++ b/src/glx/x11/Makefile
@@ -89,4 +89,4 @@
 	-rm -f *.o *~
 	-rm -f depend
 
-include depend
+-include depend