Fix for static linking

This is a small bugfix for statically linking on Windows against various platforms and plugins so they reference the proper EXPORT symbols. It slightly overloads the semantics of ${EXTRA_COMMPILE_FLAGS} but it beats adding in -DOPENMM_USE_STATIC_LIBRARIES -DLEPTON_USE_STATIC_LIBRARIES -DPTW32_STATIC_LIB staticTarget/CMakeLists.txt of every platform/plugin since they all reference EXTRA_COMPILE_FLAGS.