[physfs] Mac OS X 10.7 build problems

[Ryan C. Gordon]

> [ 14%] Building C object CMakeFiles/physfs.dir/platform/macosx.o
> cc1: warnings being treated as errors
> /Users/chrender/Downloads/swp/physfs-2.0.2/platform/macosx.c: In function ‘__PHYSFS_platformGetThreadID’:
> /Users/chrender/Downloads/swp/physfs-2.0.2/platform/macosx.c:358: warning: ‘MPCurrentTaskID’ is deprecated (declared at /System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/Multiprocessing.h:466)

These are fixed properly in revision control. Removing -Werror will work 
fine for now: these functions are no longer supported by Apple in 10.7, 
but they still work.

--ryan.