fix freebsd build

On FreeBSD, clang is the default C compiler (and the only C compiler in the base system since 10.0).
Current Go release requires explicit `CC` variable and `-extld` option to use clang instead of the nonexistent gcc.
[Docker uses nearly the same workaround](https://github.com/dotcloud/docker/blob/v0.11.1/hack/make.sh#L110).

I've also sumbitted [a pull request](https://github.com/armon/gomdb/pull/2) to armon/gomdb that adds an option required for FreeBSD support.