Re-enable Mumble support
This merge request re-enables and slightly fixes the existing Mumble support carried over from SRB2. I'm aware in internal there is work on a builtin VC solution but it would still be cool to see Mumble support re-enabled.
Only reason this seemed to be blocked off by a define is because of the WRITE* macros at the time not supporting C++. The READ*/WRITE* macros have been fixed for C++ since a4f856b0 and it seems like this define was forgotten about.
If this conflicts with internal feel free to disregard I guess?