wavpack misses ftruncate

[madebr]

Building SDL_mixer's playmus for ps2 and psp (on ci), linking fails with:

  /tmp/setupsdl/da8de0a910961c86f12e0e6d22427875f5c38084675d3be163533c2ff1367bcc/source/external/wavpack/src/open_filename.c:208:(.text+0x40): undefined reference to `ftruncate'
  collect2: error: ld returned 1 exit status

This probably means wavpack is missing a link to the math library.
Or the toolchain does not provide this symbol.

[slouken]

ftruncate is a file truncation call. If there isn't an upstream workaround, let's go ahead and disable wavpack on PS2 by default.

[sezero]

We don't use that part of the library: Can we conditionally add something, even some stub, to the library for PS? builds?

[madebr]

See libsdl-org/WavPack#2 and #586

The wavpack patch should be applied first, so ci can run on the SDL_mixer patch.