Add config MBEDTLS_SSL_SESSION_CACHE

Add configuration option MBEDTLS_SSL_SESSION_CACHE to control
enabling/disabling of the cache based session resumption.
diff --git a/programs/ssl/ssl_client2.c b/programs/ssl/ssl_client2.c
index 9828576..a3f5d60 100644
--- a/programs/ssl/ssl_client2.c
+++ b/programs/ssl/ssl_client2.c
@@ -2545,12 +2545,14 @@
             }
         }
 
+#if defined(MBEDTLS_SSL_SESSION_CACHE)
         if( ( ret = mbedtls_ssl_set_session( &ssl, &saved_session ) ) != 0 )
         {
             mbedtls_printf( " failed\n  ! mbedtls_ssl_set_session returned -0x%x\n\n",
                             -ret );
             goto exit;
         }
+#endif /* MBEDTLS_SSL_SESSION_CACHE */
 
         if( ( ret = mbedtls_net_connect( &server_fd,
                         opt.server_addr, opt.server_port,