win32 integration

These are the necessary changes to get this addon in shape for win32. The first commit is straight forward but the second might be a bit hacky. On win32 we also need to link against libmpghip and not just libmp3lame so I added another file to find that library but it doesn't have include files. Leaving the find_path() call out resulted in a CMake error so I just added a dummy call there. Maybe @notspiff has some better approach?