Get to a state where `pod lib lint` succeeds.

I'm not convinced it's actually building at this point. But I think I've figured
out a lot of the problem.

Right now it's attempting to compile header files with the source files. At least
that's what my interpretation of those build NOTES I'm seeing.

I'm having prepare_command delete files I don't want built so I can eventually
transition the libgit2/ dir to a submodule.