Refresh config.rpath for midnightbsd support
Refresh config.rpath from gnulib upstream (so far we've always updated it from gettext when there's a new gettext release). First to S3_1 & master, probably to S3_0 and S2_6 too after 3.0.0-beta1 and 2.6.4 releases.
The update brings midnightbsd support.
Pushed to master & S3_1
Pushed to S3_0, leaving only S2_6