--- swipl-9.3.24/packages/xpce/src/h/syntax.h.orig
+++ swipl-9.3.24/packages/xpce/src/h/syntax.h
@@ -62,6 +62,9 @@
 #define QT	makeCFlag(10)		/* String quote (context: escape) */
 #define PU	makeCFlag(11)		/* Punctuation */
 #define EB	makeCFlag(12)		/* End Buffer/string */
+#if (defined  __illumos__) && (defined CS)
+#undef CS
+#endif
 #define CS	makeCFlag(13)		/* Comment-start (context: 2nd char) */
 #define CE	makeCFlag(14)		/* Comment-end (context: 2nd char) */
 #define CT	makeCFlag(15)		/* Control character */