Since this gets used by both the sound backend and the music code it needs to be in a place accessible to both.
# Conflicts:
# src/CMakeLists.txt
# Conflicts:
# libraries/zmusic/decoder/mpg123_decoder.cpp
# libraries/zmusic/decoder/sndfile_decoder.cpp
# src/CMakeLists.txt
This was getting a bit unwieldy. The include path setup is not perfect yet, that's work for later.
(It's about time we're getting C++20 with modules so that this include path madness can be put to an end.)
