Makefile: run $(MAKE) -f Makefile.mak using -e

Makefile: run $(MAKE) -f Makefile.mak using -e

Explicit command line variable settings are supposed to override
Makefile variables.  However, GNU make fails to correctly pass these
overrides along to sub makes like it should.

It does, however, set them in the environment like it's supposed to
so run the "Makefile.mak" make using the "-e" option so that the
correct behavior is obtained when GNU make is "make".

While this does mean environment variables in general will override
Makefile variables of the same name, problematic variables can be
handled by "Makefile.sh" if necessary since it runs before the "make"
using "Makefile.mak" does.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>