math/mpfr: Restore to 4.1.1

Unfortunately I managed to include a partial patch to be submitted.
Restore to 4.1.1 and set PORTEPOCH. Pointy hat to myself :-/
This fixes commit 12f1c627d4a26e8098339900303640f7520b6673

Reported by: VVD
Approved by: portmgr (blanket, build fix)