furnace/extern/mpg123/ports

This directory used to contain some (contributed) works covering usage and/or
building of mpg123/libmpg123 from different programming environments. Since
they became too outdated, all but the CMake port were removed.

The CMake build is primarily intended for platforms not well-supported by the
primary autotools-based build. It is tested occasionally, had some tuning
over time, but may get out of sync regarding possible build configurations.