no method signature for unimplemented optional method

DO NOT MERGE (but do read)

For ext_globalMethodSignatureForSelector(), when a protocol has an optional method that is not implemented anywhere, no method signature is returned. **NOT A FIX**: This is only a failing test showing this.

I can try a fix you you guys think this is a valid problem (it's a problem for one of my crazy-ass use cases!).

The first commit is a not-for-merge hack to fix the build, needed to get this off the ground, per https://github.com/jspahrsummers/libextobjc/issues/41.