On FreeBSD, 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`.
Fix stripping away non-digits from OS release number
Merge pull request #630 from sstephenson/freebsd-make

On FreeBSD 10, stop defaulting to MAKE=gmake