Rebar does not support $MAKE (to my knowledge), so use a shim makefile

GNU make will look for GNUmakefile before Makefile, so move the makefile
over there and add a stub Makefile that simply defines the target that
rebar tries to build. This fixes the case where make is not gnu make.
Merge pull request #1 from Vagabond/adt-freebsd-support

Rebar does not support $MAKE (to my knowledge), so use a shim makefile