commit a patch to our fribidi cmake, from downstream (macports)

see here:
https://trac.macports.org/browser/trunk/dports/games/wesnoth/files/patch-cmake-FindFriBiDi.cmake.diff

i tested that cmake still works and finds fribidi for me (linux)
Merge pull request #317 from cbeck88/patch_cmake_fribidi

commit a patch to our fribidi cmake, from downstream (macports)