Refactor SimpleDatabase::GetLastModified() to be generic for all
plugins.  Remove the SimpleDatabase assumption from db_stats_print(),
allowing it to be implemented by all database plugins.

Fixes crash on "stats" in certain configurations.

Eliminate redundant declaration.

Unused outside of Stats.cxx.

If no music_directory is configured, return nullptr instead of an
empty string.  This fixes a crash when db_file is configured without
music_directory.

Fixes build tests on ia64 and mipsel by testing for endianness in a
more general / portable way.

Improve readability.

The option "--enable-fluidsynth" was documented to be "auto" by
default, when it was really "no".

Just an attempt to prevent users from shooting themselves in the foot
by uncommenting these lines without knowing what they're doing.

Not supported by libc++.

Required for building with libc++.

bzip2-compressed tarballs are obsolete, xz is better and widely
understood.

Commit 77c63511 caused MPD to become stuck right after a song change.
The problem was that at some point, the MusicBuffer became full, and
the DecoderThread working on the next song waits for the PlayerThread.
However, the PlayerThread was stuck in a loop of g_usleep() calls, and
never bothered to tell the DecoderThread that the MusicBuffer is not
full anymore.  This bug is very old, but its chance to occur went from
nearly 0% to nearly 100%.

The fix is to wake up the DecoderThread before waiting for it.  As a
side effect, I replaced the g_usleep() call with a Cond::Wait() call.

Remove the "do_wait" parameter which is always true.  Check only
command==NONE and merge the "return" statements.

Nothing of interest happens here.  No need to wake up the player.