Allow using the system provided libgit2.

This adds a `--use-system-libraries` to the `extconf.rb` file. This allows Rugged to be built with the system provided libgit2 installation, instead of using the vendored version.

I'm using the `LIBGIT2_SOVERSION` to check for API compatibility. Right now, this is hard-coded into the `extconf.rb` file, but I think we could also just pull the version out of the bundled libgit2 version and compare the two.

This should fix https://github.com/libgit2/rugged/issues/368.