--- swipl-8.5.17/src/pl-arith.c.orig	2022-09-19 12:59:10.000000000 +0000
+++ swipl-8.5.17/src/pl-arith.c	2022-09-23 09:14:26.915344393 +0000
@@ -2867,6 +2867,7 @@
 #define IEEE754 1
 
 #ifdef IEEE754
+/* we have signbit defined in iso/math_c99.h:
 static inline int
 signbit(double f)
 { union
@@ -2877,6 +2878,7 @@
   v.f = f;
   return v.i < 0;
 }
+*/
 
 #ifndef copysign
 double