Avoid failed include when Makefile is processed with no target.

Basically just a warning, but disconcerting nonetheless. Removes this
output from the build:
Makefile:59: -x86_64-darwin13-gcc.mk: No such file or directory

Change-Id: Ibb379506352b2f613ef4a7b1ac47e9c95d0d1580
1 file changed