FindMPI: Parse '-l:' flags in pkg-config output

With the msmpi package in MSYS2:

    $ pkg-config --libs msmpi
    -LC:/msys64/mingw64/lib -l:libmsmpi.dll.a

MSYS2-Issue: https://github.com/msys2/MINGW-packages/pull/11839
Fixes: #23620