Description of problem:

Fortran/C interface detection is broken when LTO is enabled under gcc12, which is in rawhide. cmake-based packages using this interface are currently FTBFS in rawhide (e.g.: https://koschei.fedoraproject.org/package/flexiblas?collection=f36).


How reproducible:

    $ cat << EOL > CMakeLists.txt
    PROJECT(F-C-MWE LANGUAGES C Fortran) 
    include(FortranCInterface)
    FortranCInterface_HEADER(FCMangle.h
                            MACRO_NAMESPACE "FC_"
                            SYMBOL_NAMESPACE "FC_"
                            SYMBOLS mysub mymod:my_sub)
    FortranCInterface_VERIFY()
    EOL

    $ mkdir -p build && cd build
    $ export FFLAGS='-flto'
    $ cmake ..


Actual results (some lines suppressed for legibility):

    -- Detecting Fortran/C Interface - Failed to recognize symbols
    CMake Warning (dev) at /usr/share/cmake/Modules/FortranCInterface.cmake:309 (message):
    No FortranCInterface mangling known for mysub

    CMake Warning (dev) at /usr/share/cmake/Modules/FortranCInterface.cmake:295 (message):
    No FortranCInterface mangling known for mymod:my_sub

    -- Verifying Fortran/C Compiler Compatibility
    CMake Warning (dev) at /usr/share/cmake/Modules/FortranCInterface.cmake:309 (message):
    No FortranCInterface mangling known for VerifyFortran

    -- Verifying Fortran/C Compiler Compatibility - Failed
    CMake Error at /usr/share/cmake/Modules/FortranCInterface.cmake:391 (message):

        /usr/bin/ld: /tmp/cclMYidY.ltrans0.ltrans.o: in function `main':
        /usr/share/cmake/Modules/FortranCInterface/Verify/main.c:14: undefined reference to `VerifyFortran'
        collect2: error: ld returned 1 exit status


Expected results: Detection succeeds.