--- plplot-5.9.9/src/plbox.c	Thu Oct 13 02:43:01 2011
+++ plplot-5.9.9/src/plbox.c.1	Sat May 27 17:53:10 2017
@@ -1817,6 +1817,9 @@
 // Writes numeric labels on side(s) of box in custom locations
 //--------------------------------------------------------------------------
 
+extern void plwxtik( PLFLT x, PLFLT y, PLBOOL minor, PLBOOL invert );
+extern void plwytik( PLFLT x, PLFLT y, PLBOOL minor, PLBOOL invert );
+
 void
 label_box_custom( const char *xopt, PLINT n_xticks, const PLFLT *xticks, const char *yopt, PLINT n_yticks, const PLFLT *yticks )
 {