diff --git a/libm/freebsd-compat.h b/libm/freebsd-compat.h
index f1985d2..9555ff9 100644
--- a/libm/freebsd-compat.h
+++ b/libm/freebsd-compat.h
@@ -16,6 +16,10 @@
 
 #pragma once
 
+// Since we're implementing all the extensions,
+// we need to make sure we get all their declarations when we include <math.h>.
+#define _BSD_SOURCE
+
 // Some FreeBSD source includes <complex.h> and assumes <math.h> from that.
 #include <math.h>