Prepend the vendored libgit2 build dir to the $DEFLIBPATH.

By default, $DEFLIBPATH includes $(libpath), which usually points
to something like /usr/lib for system ruby versions (not those
installed through rbenv or rvm).

This was causing system-wide libgit2 installations to be preferred
over of our vendored libgit2 version when building rugged.

By putting the path to the vendored libgit2 library at the front of
$DEFLIBPATH, we can ensure that our bundled version is always used.

This supersedes #352 and #354, and should really fix #351.