*: 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/config/Block.hxx

@@ -44,13 +44,10 @@ struct BlockParam {
 	BlockParam(const char *_name, const char *_value, int _line=-1)
 		:name(_name), value(_value), line(_line), used(false) {}
 
-	gcc_pure
 	int GetIntValue() const;
 
-	gcc_pure
 	unsigned GetUnsignedValue() const;
 
-	gcc_pure
 	bool GetBoolValue() const;
 };
 
@@ -116,13 +113,10 @@ struct ConfigBlock {
 	AllocatedPath GetPath(const char *name,
 			      const char *default_value=nullptr) const;
 
-	gcc_pure
 	int GetBlockValue(const char *name, int default_value) const;
 
-	gcc_pure
 	unsigned GetBlockValue(const char *name, unsigned default_value) const;
 
-	gcc_pure
 	bool GetBlockValue(const char *name, bool default_value) const;
 };