UseSWIG: Add -dllimport if has not already been specified

SWIG fails with a redefined macro error when -dllimport is specified more than once. One case where you would want to specify your own dllimport is when using SWIG with netstandard2.0. .NET Core does not search for shared library like Mono or .NET, so you need a fully qualified shared library name.

Topic-rename: UseSWIG-avoid-dllimport-repeat