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.

There is not much use in GLib: on Windows, we have a native API for
string collation, and the rest uses either libicu or the standard C
library calls.

We don't need this anymore: Win32 doesn't use this library at all, and
everything else has either iconv() or libicu.

New in clang 3.6.

Prepare for a ICU-based backend without GLib.

Again, POSIX says strcasecmp is not in string.h.

Make valgrind happy.