Allow use of constexpr (where supported) via a macro.

This partially reverts c06667c4828cb7623f46e3c45a971f447c1f9394 but with
adaptations.
Make OffMeshConnections::MAX_CON constexpr.
UpdateReverbSlot: check slot no. and ratio.
Don't trust the reported data chunk size in WAV files.

This is not a full fix. Checking against (length - offset) is needed.