snes_spc source from https://github.com/jprjr/snes_spc

Sync to version 187843d (12. March 2021)

Original source:
- http://www.slack.net/~ant/libs/audio.html#snes_spc
  - http://blargg.parodius.com/libs/snes_spc-0.9.0.zip (17. Apr 2007)