don't hard-code commands to gmake

It's easier to just create a `make` symlink to `gmake` in the build
slave. This also follows the same pattern we used for Solaris builds.
remove unused software definitions

`libffi` and `libexecinfo` are no longer needed for FreeBSD builds so
we'll just remove them so they don't bit rot.
format/style changes
remove FreeBSD specific ENV vars and configure commands
remove trailing whitespace