allocation hooks...

[Ryan C. Gordon]

This was less important in terms of desired features, but it touches a
lot of code and overlaps with some other things that need doing for 2.0,
so I figured I'd knock this out first.

Initial structure is in place for the allocation hooks:

  svn diff -r656:657 svn://svn.icculus.org/physfs/

I hooked this up in posix.c as a basic malloc/realloc/free passthrough,
and wired it up to zlib, just to see if it was possible. External API is
filled in.

The head revision won't build on non-Unix platforms right now because it
doesn't have the new platform functions filled in at the moment. Sync to
revision 656 or use branches/stable-1.0 if you need the latest and
greatest that compiles everywhere.

--ryan.