Add FreeBSD submodule using gmake

This changes the command "make" for freebsd to be "gmake".

The same problem surely occurred with the other BSDs which are failing tests on cpantesters, so it would be better to make the whole BSD module do this. However there is no BSD module yet. Perhaps this can be sent to cpan testers for the next trial version, and if successful then create the whole BSD module for all the BSDs. I think the use of "gmake" is common to all the systems.

Also "gmake" may not be installed on some FreeBSD systems. It usually will get sucked in via installing a port or something, but it would be sensible to actually test there is a gmake command and print out a message if the gmake cannot be found.