Fixes regression from commit 1473d847

Fixes regression by commit 23d5a2b8 -
that commit always pretended that any Opus file has both track and
album gain, and thus disabled the fallback to the other if one is not
set.

This patch changes the logic to only submit ReplayGain if at least one
value is set, and apply the offset only to that value.  If none is
available, then the new check in HandleAudio() will submit only the
output gain.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/977

Closes https://github.com/MusicPlayerDaemon/MPD/issues/979

Keep the logging library out of this low-level libcurl wrapper.

We need at least GCC 8 anyway.

Fixes regression by commit 521e573b

Fixes regression by commit 7901b04c