Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Closes #34

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

Forward the configured ReplayGainMode to all outputs and decoders.

Yet another global variable removed.

Obsoletes the same variable from AudioConfig.cxx.

Remove dependency on ReplayGain global variables.

Move code from replay_gain_get_real_mode().

Use MaskMonitor to eliminate duplicate code.