*: remove "pure" and "const" attributes from throwing functions

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.

src/AudioParser.hxx

@@ -25,8 +25,6 @@
 #ifndef MPD_AUDIO_PARSER_HXX
 #define MPD_AUDIO_PARSER_HXX
 
-#include "Compiler.h"
-
 struct AudioFormat;
 
 /**
@@ -38,7 +36,6 @@ struct AudioFormat;
  * @param src the input string
  * @param mask if true, then "*" is allowed for any number of items
  */
-gcc_pure
 AudioFormat
 ParseAudioFormat(const char *src, bool mask);