Detect GNUmakefile too

Hi. The buildpack doesn't check for the existence of a GNUmakefile, which is a valid name for a makefile under GNU Make, but specific to GNU Make. A reason to use this file is, for example, if a project uses GNU Make extensions and a wishes to provide only GNUmakefile and leave the chance to create a more portable Makefile later.

According to the manual, GNU Make looks for Make files in the following order: GNUmakefile, makefile, Makefile.

I'm not sure if this is the correct fix and it is untested. What do you think? Comments and rejections welcome. Thanks.