FortranCInterface: Re-runs on every configure since 3.21
Moving report from original description of !6581 (merged).
A regression in CMake 3.21 causes the Fortran/C interface detection to be re-run at every configure:
-- Detecting Fortran/C Interface
-- Detecting Fortran/C Interface - Found GLOBAL and MODULE mangling
because an input file is incorrectly expected to be in the build directory.
This was broken by refactoring in commit d7c80305 from !5825 (merged).