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 unshift the vendored libgit2 path to the front of the
$DEFLIBPATH, which ensures that the vendored version is preferred over
any system-wide libgit2 installations.