add warning to if-statement in check mode

warning is shown if the condition of an
if statement can never be true or false,
and the user used the check command.
warnings are not shown when you use `mers run`.

mers/src/main.rs

@@ -125,7 +125,8 @@ fn main() {
                     exit(20);
                 }
                 Ok(parsed) => {
-                    let (i1, _, i3) = config.infos();
+                    let (i1, _, mut i3) = config.infos();
+                    i3.global.show_warnings_to_stderr();
                     match compile(&*parsed, i1) {
                         Err(e) => {
                             eprintln!("{e:?}");