Remove GNU-specific code from the Makefile.

The Makefile is now compatible with BSD make, at least the version that
ships with FreeBSD 10.1. Trying to be compatible with both GNU and BSD
make has uglied up the Makefile considerably. I may switch back to
GNU-only at some point.
Don't override MAKE.

Both GNU and BSD make define $MAKE appropriately, and explicitly
overriding it in the Makefile just makes it annoying to use gmake on
BSD. You can always specify MAKE= on the command line if you need to
override it for some reason.
Remove the tmp.grand.test target.
Make the do.grand.test target depend on ${SCHEME}.

This removes the need to separately build the interpreter before running
``make grand.test''.