On FreeBSD 10, stop defaulting to MAKE=gmake

It appears that regular `make` that ships on FreeBSD 10 is compatible enough to build Rubies. This enables ruby-build on fresh FreeBSD installs (which don't have `gmake` by default) without having to explicitly set `MAKE=make`.

Fixes #496, references #293

/cc @onibox @caleb @shugo