Several weird issues with this one:

- It tries to find libid3tag and libmad via pkg-config even though they
  don't install .pc files.  Perhaps we can generate them manually in the
  install phase of those packages, or maybe they just don't get
  installed due to a bug.  Perhaps they just need 'pkg-config' as an
  input?  I might try that next.

- Audacity apparently uses PortAudio version 19, but the in-tree one has
  a function which the upstream one doesn't.  It would be strange if
  they had an API change without changing the version, so no idea what's
  going on there.

- Dynamic loading of FFmpeg fails and I couldn't figure out why (it
  didn't print any errors when I ran Audacity and normally it's pretty
  verbose) so I just passed --disable-dynamic-loading and it was fine.

- The test suite fails due to some missing portaudio.h file.  I didn't
  come far trying to figure out why this happens so I just disabled the
  test suite.

I might try looking into all these further, but they aren't critical to
Audacity's working.  Decoding MP3 works fine without libmad/libid3tag.