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

Closes #169

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.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Move code from SongFilter.cxx.

New in clang 3.6.

According to POSIX and both OSX and Linux manpages,
strcasecmp comes from strings.h, not string.h.

Most OSes also have them available in string.h,
but we just fixed the headers on Haiku and it now
only provides them in strings.h.

We might want to fall back to string.h for other
OSes though...

cf.
http://pubs.opengroup.org/onlinepubs/009695399/functions/strcasecmp.html
http://linux.die.net/man/3/strcasecmp
https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man3/strcasecmp.3.html

Replaces GLib's g_ascii_strncasecmp().

Replaces GLib's g_ascii_strcasecmp().