Add define for GNU glibc extensions

`fdopen`, `strdup`, and `mkstemp` are all extensions. `_DEFAULT_SOURCE`
should be enough for this, but apparently not on older glibcs.

Author: bnbarham

api_test/harness.c

@@ -1,4 +1,5 @@
 #define _DEFAULT_SOURCE
+#define _BSD_SOURCE
 #include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>