GNU Make 3.x doesn't support the "!=" syntax for shell assignment,
although 4.x does, BSD Makes do, and it has been submitted to POSIX
for possible inclusion in the standard. Unfortunately, GNU Make 3.x
is still in common use, so provide an alternative makefile (GNU Make
reads "GNUmakefile" instead of "Makefile" by default) that sets the
variables and then reuses the rest of the default Makefile.
