The clang/libc++ version in the default xcode8 image is unable to
compile MPD due to incomplete C++14 support.

The normal I/O event thread can have a large latency, e.g. when
libgnutls loads all TLS CA certificates for a https connect.  This
makes it unreliable for the ALSA I/O notifications, and causes ring
buffer xruns.  To avoid interfering with high latency events such as
CURL's, we move the ALSA I/O events to a separate I/O thread which
also obtains real-time scheduling (if possible).

Closes #221

release v0.20.17

Due to rounding errors, a slightly negative value can be passed to
set_normalized_volume(), which will make the log10() call fail.
Actually, volume 0 is already failing because log10(0) is illegal.  So
let's fix this by implementing two corner cases: <=0 and >=100.

Closes #212

Closes #213.

Apparently, clang defaults to NEON when ARMv7 is used.  Not all ARMv7
CPUs we target have NEON, so we need to disable that.

Not needed on Android, and the implementation uses getpwuid_r() which
is unavailable on old Android versions.

Requires Android API 21, but we want to support older versions as
well.

For Android pre-5.0 compatibility (#213).

For Android pre-5.0 compatibility (#213).

For version numbers such as OpenSSH's, e.g.: "7.2p2"

Closes #217

Fixes valgrind warning.

This was a buffer overflow bug which could cause MPD crahes when
playing back 8 channels with the ALSA output plugin.

Closes #216