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/ssl_mail_client.c b/programs/ssl/ssl_mail_client.c
index 205c93e..24000a2 100644
--- a/programs/ssl/ssl_mail_client.c
+++ b/programs/ssl/ssl_mail_client.c
@@ -662,11 +662,13 @@
         goto exit;
     }
 
+#if !defined(MBEDTLS_X509_REMOVE_HOSTNAME_VERIFICATION)
     if( ( ret = mbedtls_ssl_set_hostname( &ssl, opt.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) &&          \