update csound to 6.15.0.

changes from 6.14.0 include:

- new opcodes: randc, mp3out, metro2, ftexists, schedulek,
  lastcycle, and strstrip
- new array based channel opcodes: chngeti, chngetk, chngeta,
  chngets, chnseti, chnsetk, chnseta, and chnsets
- terniary expressions only evaluate the chosen path now
- orchestra macros persist in more places
- modified opcodes: squinewave, pindex, sflooper, event_i,
  fluidControl, bpf, stsend, stecv, pvstrace, lpfreson,
  syncloop, bpfcs, zacl, outvalue, chnget, chnset, copya2ftab,
  syncloop, lowresx, and outch
- // style comments allowed

changes from 6.15.0 include:

- new opcodes: ftset, lufs, bob (Moog analog based filter),
  sterrain, wterrain2, count, count_i, cntCreate, cntRead,
  cntReset, cntCycles, cntState, println, rndseed, arduinoStart,
  arduinoRead, arduinoStop, lpcfilter, lpcanal, allpole, pvslpc,
  pvscfs, apoleparams, resonbnk, gauss, pvsbandwidth, and vps
- many orchestra fixes
- modified opcodes: cent, semitone, taninv2, ftslice, ptable,
  GEN20, fprint(k)s, lastcycle, chn_k, trim, GEN16, scale,
  schedule, schedulek, GEN11, partials, ctrlinit, fin, and fink.
- many bugs fixed

this also fixes the build which broke recently for reasons i
can't quite figure out.  also PR#55696.

(mrg)