*: 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/protocol/ArgParser.hxx

@@ -21,7 +21,6 @@
 #define MPD_PROTOCOL_ARGPARSER_HXX
 
 #include "check.h"
-#include "Compiler.h"
 
 #include <limits>
 
@@ -30,15 +29,12 @@
 class SongTime;
 class SignedSongTime;
 
-gcc_pure
 uint32_t
 ParseCommandArgU32(const char *s);
 
-gcc_pure
 int
 ParseCommandArgInt(const char *s, int min_value, int max_value);
 
-gcc_pure
 int
 ParseCommandArgInt(const char *s);
 
@@ -55,31 +51,24 @@ struct RangeArg {
 	}
 };
 
-gcc_pure
 RangeArg
 ParseCommandArgRange(const char *s);
 
-gcc_pure
 unsigned
 ParseCommandArgUnsigned(const char *s, unsigned max_value);
 
-gcc_pure
 unsigned
 ParseCommandArgUnsigned(const char *s);
 
-gcc_pure
 bool
 ParseCommandArgBool(const char *s);
 
-gcc_pure
 float
 ParseCommandArgFloat(const char *s);
 
-gcc_pure
 SongTime
 ParseCommandArgSongTime(const char *s);
 
-gcc_pure
 SignedSongTime
 ParseCommandArgSignedSongTime(const char *s);