Have c_parse_makefile look for GNUmakefile as well
Have c_parse_makefile look for GNUmakefile as well

Makefiles using GNU-make features might be called "GNUmakefile" instead
of "Makefile". This commit teaches the `c_parse_makefile` feature to
look for a GNUmakefile file if a Makefile is not present.

w0rp/ale,w0rp/ale,IngoHeimbach/ale,IngoHeimbach/ale,w0rp/ale,IngoHeimbach/ale