[physfs] MinGW GCC 4 Changes required

[Ryan C. Gordon]

> I failed to build PhysFS from source with MinGW GCC 4.4.0
> The following changes worked which was suggested by: 
> http://supertux.lethargik.org/wiki/Building_on_Windows

This is now fixed in the default and stable-2.0 branches (stable-1.0 
doesn't use CMake).

Thanks!

--ryan.