Remove dependency to CPP: support for #include directive

The parser is prepared to handle '#' directives, starting with file
inclusion. The search path for the file are taken from what was
actually given to CPP. There is an arbitrary limit to the inclusion
nesting, which is actually not a design limitation but a security
to avoid infinite include loops.

WINGs/menuparser.h

@@ -28,8 +28,12 @@
  */
 
 #define MAXLINE              1024
+#define MAX_NESTED_INCLUDES  16  // To avoid infinite includes case
 
 struct w_menu_parser {
+	WMenuParser include_file;
+	WMenuParser parent_file;
+	const char *include_default_paths;
 	const char *file_name;
 	FILE *file_handle;
 	int line_number;