FortranCInterface: Fix regression in timestamp check
Since commit d7c80305 from !5825 (merged), FortranCInterface checks for Output.cmake.in
in the build tree instead of the source tree as before. This caused it to always re-run the detection.
Fixes: #22709 (closed)
Backport: release
Topic-rename: FortranCInterface-timestamp-check
Edited by Brad King