PhysFS 2.0 only offers PHYSFS_read/PHYSFS_write for I/O. PhysFS 3.0
deprecates PHYSFS_read / PHYSFS_write and offers PHYSFS_readBytes /
PHYSFS_writeBytes. Converting Rebirth to use the new API is somewhat
invasive, and would require dropping support for PhysFS 2.0.
For now, for compatibility with PhysFS 2.0, disable the deprecation
errors and continue to use the older functions.
Reported-by: gabeotisbenson <https://github.com/dxx-rebirth/dxx-rebirth/issues/352>