Make function mbedtls_ssl_set_hostname(...) as optional

Now function mbedtls_ssl_set_hostname is compile-time configurable
in config.h with define MBEDTLS_X509_REMOVE_HOSTNAME_VERIFICATION.
This affects to many x509 API's. See config.h for details.
diff --git a/programs/ssl/dtls_client.c b/programs/ssl/dtls_client.c
index 39a9f76..b31090f 100644
--- a/programs/ssl/dtls_client.c
+++ b/programs/ssl/dtls_client.c
@@ -222,12 +222,13 @@
         mbedtls_printf( " failed\n  ! mbedtls_ssl_setup returned %d\n\n", ret );
         goto exit;
     }
-
+#if !defined(MBEDTLS_X509_REMOVE_HOSTNAME_VERIFICATION)
     if( ( ret = mbedtls_ssl_set_hostname( &ssl, SERVER_NAME ) ) != 0 )
     {
         mbedtls_printf( " failed\n  ! mbedtls_ssl_set_hostname returned %d\n\n", ret );
         goto exit;
     }
+#endif /* !MBEDTLS_X509_REMOVE_HOSTNAME_VERIFICATION */
 
 #if !defined(MBEDTLS_SSL_CONF_RECV) && \
     !defined(MBEDTLS_SSL_CONF_SEND) && \