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