Force use of GNU Make on FreeBSD

The Makefile is not compatible with BSD make, and will produce the
following error when used on FreeBSD:

  Error expanding embedded variable.
  ERROR: Command [compile] failed!

Also, os:type/0 will probably never return a generic "bsd" as the
operating system: according to os(3), the value from "uname -s" will be
used.
Merge pull request #77 from olgeni/bsdmake

Force use of GNU Make on FreeBSD