rename Makefile to GNUmakefile

This fixes a mysterious bug, where gnu make goes into an infinite loop,
calling itself.  The command line was

   $(MAKE) --warn-undefined-variable --no-builtin-rules -f Makefile.build "$@"

, but the -f option was ignored, and instead of reading Makefile.build
it read Makefile.  I have no idea why renaming it fixes it.