[physfs] Build issues with physfs 2.0 / HG

[Ryan C. Gordon]

> to check for __unix__ || unix.

Fixed now.

> HG has also added a section for BSD OS's but it checks for the
> symbol BSD which is only defined if sys/param.h is included

I ended up plugging in the common BSDs instead.

> The other issue with 2.0 was with the __PHYSFS_platformGetThreadID()
> function failing to build as is. This commit needs to be back ported
> to the 2.0 branch..

Done.

Let me know if this gets you running.

--ryan.