FindMPI: Don't retain libdirs for Win32

On Windows systems, the need to retain library directories should not nomrally occur. This can only occur on Unix like systems where a shared library is being linked that has a dependency on another library which is not being found in the default directories (e.g. libgfortran on BSD)

This also works around an issue introduced by !1694 (merged) in that the Visual Studio generator does not handle link flags passed in INTERFACE_LINK_LIBRARIES correctly, causing problems when using that generator with Intel MPI in certain circumstances.