configure.ac: prepare for 0.19.6
.gitignore: ignore /lib/
android: update libFLAC to 1.3.1

Due to security vulnerabilities.
android/build.py: update ffmpeg 2.4.3, curl 7.39
Output: start with a null mixer.

There are code paths (mostly error cases) in which it is possible to
initialize an AudioOutput and then kill it without ever calling
audio_output_new().  In such a case, its destructor will attempt to
free a mixer that was never initialized, leading to an attempt to
take out a lock on a mutex that was similarly never initialized,
which hangs forever.

Fix by always initializing the mixer appropriately.
command: use ConstBuffer<const char *> for argument list
configure.ac: prepare for 0.18.20
thread/Id: drop "::" prefix before pthread function names

The "::" to explicitly refer to the global namespace appeared like a
good idea in C++, but it breaks with C libraries that implement
standard functions using macros (e.g. musl).
decoder/ffmpeg: support FFmpeg 2.5

Version 2.5 fixed an API oddity, however it broke API compatibility,
at least with C++.  Disable the workaround when a libavformat version
is detected that is recent enough.
release v0.18.20
Merge tag 'v0.18.20' into v0.19.x
android/build.py: update FFmpeg to 2.5
release v0.19.6
Merge tag 'v0.19.6'