flac: fix building on 10.9

* Based on the work of @fernandotcl.
  - https://gist.github.com/fernandotcl/7120507

Closes #23254.
Closes #23540.

Signed-off-by: Xiyue Deng <manphiz@gmail.com>
flac: stylenit.

* Add comments for Mavericks workaround.
* Reference upstream bug report and patch.
* Use symbol form for autotools.