bulid: Rename Makefile to GNUmakefile

Prior to this commit, the build instructions were encoded in Makefiles
that contained GNU extensions that are unique to GNU Make and
incompatible with older implementations.

Thie commit renames all Makefiles to GNUmakefile which clearly denotes
that the file contains GNU extensions.

GNU Make actually first looks for a GNUmakefile, then makefile, THEN
Makefile so in addition to simply being more correct and portable, it
saves a few unnecessary failed attempts to open the correct build
file.

Author: ayang64