do not warn about missing atomic::{wait,notify}

This allows emper to be build with --meson-fatal-warnings on in our i4lab.

eval/meson.build

@@ -36,5 +36,5 @@ if cpp_has_std_atomic_wait_noitify
     dependencies: emper_dep,
   )
 else
-  warning('C++ compiler does not support std::atomic wait()/notify()/notify_all(), not building io_latency binary')
+  message('C++ compiler does not support std::atomic wait()/notify()/notify_all(), not building io_latency binary')
 endif