Fixes compiler warning because formatting unscoped enums is deprecated
since libfmt 9.

This fixes #1616

This fixes a bug introduced in 87fa6bca where the FLAC encoder fails to
initialize unless libFLAC is built with Ogg support. When libFLAC is
built without Ogg support, FLAC__stream_encoder_set_ogg_serial_number
unconditionally returns false.

Don't require "log_file" setting, for "--no-config" operation.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1600

Calling data[fill] could trigger an assertion failure if
fill==data.size(), even if we call it only to take the address.

Instead of doing that, this commit changes the code to pointer
arithmetic.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1556

See also https://github.com/MusicPlayerDaemon/MPD/issues/1061

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1570

This ensures that Partition::OnMixerVolumeChanged() invokes
MixerMemento::InvalidateHardwareVolume(), clearing the cached volume
level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1526

Avoid excessive buffering if run by CI.

Make this idle event per-partition.

Eliminate global variables, convert them to MixerMemento fields.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1583

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1529

This PR provides forward and backward compatibility at macos SDK 12.0. At SDK 12.0, API function names were changed essentially replacing
occurrences of the word Master/master with Main/main. This change was test built on two different systems.

1. macos 10.15.7 with Xcode 12.4 and clang 12.0.0 on x86_64
2. macos 12.5 with Xcode 13.4.1 and clang 13.1.6 on arm64 (Apple silicon M1)

It should be noted that on macos 10.15.7 with Xcode 11.2 and clang 11.0, MPD will not build.
The MPD documentation states that clang 11.0 is the minimum requirement,
but clang 11.0 produces compile errors. Apparently the macos version
of clang 11.0 is not fully compliant.

Commit ebae25d1 added that #include, but forgot to wrap it in
"extern C", so the linker tried to look up C++ symbols, causing linker
failure.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1582

Properly deals with iconv, unlike the current solution. have_iconv fails
when libiconv CFLAGS are passed to the compiler. Tested under OpenWrt
with its CONFIG_BUILD_NLS, which adds libiconv include flags.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Closes #1564