py: Add basic framework for issuing compile/runtime warnings.
diff --git a/py/runtime.h b/py/runtime.h
index 59d61e7..711b15f 100644
--- a/py/runtime.h
+++ b/py/runtime.h
@@ -126,4 +126,10 @@
 #define mp_sys_path ((mp_obj_t)&mp_sys_path_obj)
 #define mp_sys_argv ((mp_obj_t)&mp_sys_argv_obj)
 
+#if MICROPY_WARNINGS
+void mp_warning(const char *msg, ...);
+#else
+#define mp_warning(msg, ...)
+#endif
+
 #endif // __MICROPY_INCLUDED_PY_RUNTIME_H__