Bug 794723 - Add a default makefile which wraps mach. r=gps

This makes it easier for new contributors to get a build started
without having to know about our mach driver.

--HG--
extra : rebase_source : 19556522e80907dd9bf89d16b736ed49f7b2468a

GNUmakefile

@@ -0,0 +1,14 @@
+# This Makefile is used as a shim to aid people with muscle memory
+# so that they can type "make".
+#
+# This file and all of its targets should not be used by anything important.
+
+all: build
+
+build:
+	./mach build
+
+clean:
+	./mach clobber
+
+.PHONY: all build clean