Adapt sources to configurable config.h name

programs/ssl/ssl_client2.c

diff --git a/programs/ssl/ssl_client2.c b/programs/ssl/ssl_client2.c
index 012403a..62d29ad 100644
--- a/programs/ssl/ssl_client2.c
+++ b/programs/ssl/ssl_client2.c
@@ -23,7 +23,11 @@
  *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#if !defined(POLARSSL_CONFIG_FILE)
 #include "polarssl/config.h"
+#else
+#include POLARSSL_CONFIG_FILE
+#endif
 
 #include <string.h>
 #include <stdlib.h>