Don't use dir_config.

`dir_config` is only honoring the given include and lib paths, if
libgit2 is not found in any of the "standard" system include/lib paths.

This means that for people who had libgit2 installed on their system,
rugged would get build against their system libgit2 and not against our
bundled version.

Instead, we push the vendored libgit2 path directly into $LIBPATH, which
ensures that the vendored version is preferred over any system-wide
libgit2 installations.