Apply flag only to Fortran sources. Fix #7803.

  - Windows build had been broken for a while and I did not notice.

  - `fallow-argument-mismatch` was being applied to all source files.
    However, we have C sources and not just Fortran sources to compile.

    Apply the flag just for the Fortran sources.

    GCC and Clang on linux do not complain about this flag. But it seems
    to error out on Windows with MSYS versions of Clang.
    Maybe it is because the our MSYS clang is at 11.0 and on linux we use
    older Clang versions.