Set default MAKE=gmake on FreeBSD again

as an workaround for Ruby bug 16331: https://bugs.ruby-lang.org/issues/16331

Due to this bug, build will fail with FreeBSD's make after #1368.
The bug is already fixed in MRI upstream but GNU make is still required
when building older releases of Ruby. Use GNU make rather than switching
make/gmake depending of Ruby version.

See also: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241633