Index: cmake/config.h.in
--- cmake/config.h.in.orig
+++ cmake/config.h.in
@@ -20,6 +20,10 @@
 #define _GNU_SOURCE 1
 
 // The later definition of _XOPEN_SOURCE and _POSIX_C_SOURCE disables certain
+// features on OpenBSD, so we need _BSD_SOURCE to re-enable them.
+#define _BSD_SOURCE 1
+
+// The later definition of _XOPEN_SOURCE and _POSIX_C_SOURCE disables certain
 // features on NetBSD, so we need _NETBSD_SOURCE to re-enable them.
 #define _NETBSD_SOURCE 1
 
@@ -40,7 +44,7 @@
 
 #if defined(__SunOS_5_8) || defined(__SunOS_5_9) || defined(__SunOS_5_10)
 #  define _XOPEN_SOURCE 500
-#elif defined(__FreeBSD__)
+#elif defined(__FreeBSD__) || defined(__OpenBSD__)
 #  define _XOPEN_SOURCE 700
 #elif defined(__ibmxl__) && defined(__clang__) // Compiler xlclang
 #  define _XOPEN_SOURCE 600