Using pcm_resample_lsr_32() for 24 bit samples works, but may cause 24
bit overflows.  This commit makes 24 bit a special case with explicit
clipping.

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++.