[aquaria] [patch] CMakeLists.txt fix for proper MinGW building

[Ryan C. Gordon]

> Here's a patch that will make sure that Aquaria is built as a Windows
> GUI app instead of a console app when using GCC (and possibly MSVC).
> I find rewriting the same block into a if-else-endif redundant, but
> that's the only way it could be done. Any more efficient methods?

This doesn't work?

     IF(WIN32)
         SET(EXETYPE WIN32)
     ENDIF(WIN32)

     ADD_EXECUTABLE(aquaria ${EXETYPE} other-stuff-goes-here)

--ryan.