Fix build on FreeBSD by linking to libc++

[Freaky]

Maybe this conditional would be better off asking if we're using clang/gcc, but this fixes linking for me and allows cargo test to pass.

[Keats]

Thanks!

[Keats]

I just realised, we define a is_bsd variable above in the build. We should use that instead of just special-casing freebsd no?

[valpackett]

Assuming libc++ if clang is the compiler doesn't sound right. Assuming libc++ on any BSD… I think most have switched at this point, yeah.

I guess the smart thing would be to use the c preprocessor to check for __GLIBCXX__ and such?

[Keats]

Can I get some eyes on #32 by any chance? I'm not good with this kind of stuff