Improve setting of dllimport/dllexport on native Windows.  Allows Mingw to build with --disable-shared.
Improve setting of dllimport/dllexport on native Windows.  Allows Mingw to build with --disable-shared.

git-svn-id: 98787b36aaefbae4b30fa755be920954cd25e1e4@1499 ae7d391e-3f74-4a2b-a0db-a8776840fd2a

dcjm/polyml,polyml/polyml,dcjm/polyml,polyml/polyml,polyml/polyml,polyml/polyml,mn200/polyml,mn200/polyml,dcjm/polyml,dcjm/polyml,mn200/polyml,mn200/polyml