--- swipl-9.2.9/src/pl-arith.c.orig
+++ swipl-9.2.9/src/pl-arith.c
@@ -3059,6 +3059,7 @@
 #define IEEE754 1
 
 #ifdef IEEE754
+/* we have signbit defined in iso/math_c99.h:
 static inline int
 signbit(double f)
 { union
@@ -3069,6 +3070,7 @@
   v.f = f;
   return v.i < 0;
 }
+*/
 
 #ifndef copysign
 double