Overwrite the default $DEFLIBPATH with something more sane.

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 used
instead of our vendored libgit2 version when building rugged.

This superseeds #352 and #354, and should really fix #351.