Backport FindIntl.cmake from CMake 3.20
CMake 3.20 offers an imported target for libintl. Using it makes the code nicer and more robust and avoids issues where LibIntl_LIBRARIES is legitimately empty.
Since Frameworks won't depend on CMake 3.20 anytime soon this patch backports the file from CMake git and slightly adjusts it to work outside of the CMake source tree
Edited by Nicolas Fella