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 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.
44 jobs for findmpi-msvc-nolibdir
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