+/* Callback for header lookup for HEADER, which is the name of a
+ source file. It is used as a method of last resort to find headers
+ that are not otherwise found during the normal include processing.
+ The return value is the malloced name of a header to try and open,
+ if any, or NULL otherwise. This callback is called only if the
+ header is otherwise unfound. */
+typedef const char *(*missing_header_cb)(cpp_reader *, const char *header, cpp_dir **);
+