The needed headers are now included in the repo, which for these libraries is possible thanks to a stable ABI (at least on Windows, the other platforms still need to be checked but the headers only add, never remove or change existing content.)
The big advantage of this setup is that it allows building the project on Windows without any necessary setup - all that needs to be provided is the DLLs from the binary package.
This still requires some fixes for macOS and Linux. On MacOS the proper library names are missing and the ones for Linux are not verified. Both platforms should work, though, if the dynamic loading is disabled.
The idea is to have more control on the game side instead of dealing with these formats in the backend, which was done for FMod because it already had the decoders implemented.
However, with OpenAL this setup makes no sense and only complicates future extensions that can be better handled at a higher level.
According to MinGW headers (which is used for compiling libmpg123) ssize_t type must be 64-bit in size on 64-bit Windows
Moreover it was impossible to build GZDoom with the recent versions of libmpg123 because of wrong type redifinition
Tested MP3 music on Deus Vult II with Release x64 and OpenAL backend
Instead of the previous method where there'd be a filename and offset, and/or a
memory pointer, this uses a class to access resource data regardless of its
underlying form.
