Remove dependency to CPP: add function to report problems while parsing

The default function used so far provides informations not so useful
to user, like wmaker's source file, line number and function; it
also cannot provide the line number from the parsed file because cpp
messes this information.

With this dedicated function we try to provide useful things which
are being tracked by the parser internally, like valid line number
and the name of the file being read (which can be convenient in the
case of #include, for which we may also be able to provide the
inclusion tree!)

WINGs/WINGs/WUtil.h

@@ -874,6 +874,9 @@ typedef struct w_menu_parser *WMenuParser;
 
 WMenuParser WMenuParserCreate(const char *file_name, void *file);
 
+void WMenuParserError(WMenuParser parser, const char *msg, ...)
+	__attribute__ ((format (printf, 2, 3)));
+
 const char *WMenuParserGetFilename(WMenuParser parser);
 
 Bool WMenuParserGetLine(WMenuParser parser, char **title, char **command, char **parameter, char **shortcut);