Fix a build warning on OpenBSD
(Contributed by Theo Buehler)
diff --git a/contrib/libtests/pngvalid.c b/contrib/libtests/pngvalid.c
index e66f2fa..53170f8 100644
--- a/contrib/libtests/pngvalid.c
+++ b/contrib/libtests/pngvalid.c
@@ -21,6 +21,7 @@
#define _POSIX_SOURCE 1
#define _ISOC99_SOURCE 1 /* For floating point */
#define _GNU_SOURCE 1 /* For the floating point exception extension */
+#define _BSD_SOURCE 1 /* For the floating point exception extension */
#include <signal.h>
#include <stdio.h>