Add support for libmpg123

[SineSwiper]

This whole hunt for a good Helix MP3 decoder seems like a rather unnecessary requirement, when libmpg123 is just sitting there, licensed under the LGPLv2.1, as it has been since 2007. It's been a standard Linux MP3 decoder for three decades.

Why can't we use that?

[SineSwiper]

Ugh, I noticed the --with-libavcodec=yes in the Configure tool, after spending a few hours trying to figure out Helix support. Can we document that in the main README?

[irixxxx]

The build system is unfortunately still riddled with old cruft. I really should clean it up at some point...

Helix is only supported for armv4 to armv7 as documented in the boiler plate, and it is mandated only on GPH devices, since they are too slow for anything else. It wouldn't work on armv8+ (i.e. everything compiled for 64 bit arm).
libavcodec was mainly introduced for the Pandora device. It isn't officially used anywhere else, and support is only tested with a handful of versions (IIRC versions 52 to 55).

The fallback if neither is available is the drmp3, which is automatically compiled in if no other mp3 decoder is selected. To get there, disable libavcodec with configure, and build like this: make PLATFORM_MP3=0.