Factor and simplify Makefile's and mpconfig.
diff --git a/py/repl.h b/py/repl.h
index 02fe523..23259fa 100644
--- a/py/repl.h
+++ b/py/repl.h
@@ -1 +1,3 @@
+#if MICROPY_ENABLE_REPL_HELPERS
 bool mp_repl_is_compound_stmt(const char *line);
+#endif