strptime() on OpenBSD requires _XOPEN_SOURCE_EXTENDED 1 (fix #999)

strptime() is an XPG4v2 function, on OpenBSD the prototype
is not included in time.h unless visibility is set, it needs
either "#define _XOPEN_SOURCE_EXTENDED 1" or _XOPEN_SOURCE
>= 500. Without this the prototype isn't included and
strptime's returned pointer is truncated to an int, causing
SEGV on 64-bit arches when using it.

Thanks @sthen

src/builtin.c

@@ -1,6 +1,7 @@
 #define _BSD_SOURCE
 #define _GNU_SOURCE
 #define _XOPEN_SOURCE
+#define _XOPEN_SOURCE_EXTENDED 1
 #include <sys/time.h>
 #include <stdlib.h>
 #include <stddef.h>