Allow selection of make with ${MAKE} and suggest sensible defaults for B...

The following diff allows user to set the `MAKE` environment variable to override the make implementation used. In it's absence, a sensible default will be chosen based upon the platform (HippyVM needs GNU make, which is called `make` on Linux systems and `gmake` on BSD systems).

I did consider adding a `platform` module like in PyPy, but since this is the only place we need this, I opted to make the change inline.

This at least allows RPython to emit C code for HippyVM on my OpenBSD system. The C code does not yet build on my system, but that is a separate problem (I am looking into that).