_BSD_SOURCE is deprecated, use _DEFAULT_SOURCE instead

The feature_test_macros(7) manual tells us that _BSD_SOURCE is
deprecated since glibc 2.20 and that the compiler will warn about it
being used, unless _DEFAULT_SOURCE is defined as well.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(cherry picked from commit f9fd35248c)

ssl/ssltest.c

@@ -116,6 +116,7 @@
 
 /* Or gethostname won't be declared properly on Linux and GNU platforms. */
 #define _BSD_SOURCE 1
+#define _DEFAULT_SOURCE 1
 
 #include <assert.h>
 #include <errno.h>