[physfs] More small fixes

[Ryan C. Gordon]

> physfs-static.diff:
>  - Add OPTIONAL_LIBRARY_LIBS to the static version of physfs (so -lz
> can be used at linking stage for test_physfs).
>  - Use TARGET_SUPPORTS_SHARED_LIBS instead of TRUE, so platforms with
> only static linking will not try to build shared versions.

I didn't apply this diff because you said it was broken. If that 
changes, let me know.

> physfs-threads.diff:
>  - CmakeList.txt define or not PHYSFS_NO_THREAD_SUPPORT for thread
> support, but src/platform_unix.c checks for PHYSFS_NO_PTHREADS_SUPPORT. 

This is now hg changeset 1079:c218c384331e, thanks! (it was already in 
the stable-2.0 branch, I must have missed it for the development branch).

--ryan.