pjsip: new formula

I finally got around to making a pjsip formula that works, with much trial and error!

I haven't quite figured out why I have to manually set `CROSS_COMPILE` to an empty string, since I don't have to do this if I do the exact same thing outside of homebrew. I've also tried using `--env=std`, no difference. However, it seems like this isn't a problem with the current `HEAD`, which will be released as `2.2`.

It also includes two tiny patches to make pjsip compile with clang, this is also included in the current `HEAD` and will be in `2.2`.