Define _BSD_SOURCE in builtin.c for OpenBSD support (#3278)

On OpenBSD, some prototypes are hidden behind `_BSD_SOURCE`.
Defining this variable fixes #3252, and also fixes #3276.
This can be seen as a regression of #2631.

src/builtin.c

@@ -5,6 +5,9 @@
 # define _XPG6
 # define __EXTENSIONS__
 #endif
+#ifdef __OpenBSD__
+# define _BSD_SOURCE
+#endif
 #include <sys/time.h>
 #include <stdlib.h>
 #include <stddef.h>