Fix various errors and warnings that prevented build assimp_view as linux to win32 cross compile.

Hello,

this pull request contains various minor changes allow to build the assimp_viewer on linux for win32. This allows for example to use the assimp_viewer with wine.

I would also like to know if there are any plans for the future of the assimp_viewer or if it should be dropped in the long term. If this isn't the case and if there aren't any objections to this I would like to replace the current win32 implementation with a QT based backend to make the viewer available on more platforms.