cmStandardLevelResolver: Avoid unnecessary flags, fix unset level logic

The changes are part of CMP0128.

When the standard level is unset:
* Flags are added if extension mode doesn't match the compiler's default.
  Previously logic only worked if LANG_EXTENSIONS was ON. Fixes #22224.
* The full flag is used. Previously CMAKE_LANG_EXTENSION_COMPILE_OPTION was
  used. This was only supported for IAR.

Otherwise:
* Avoid adding flags if not necessary per the detected compiler defaults.
* Fixed check for when the requested standard is older. It now matches the
  nearby comments.

I reworded the fallback comment as its logic was a bit difficult to wrap my
head around.

1 files changed, 1 insertions, 1 deletions

diff --git a/Tests/RunCMake/CompileFeatures/empty.c b/Tests/RunCMake/CompileFeatures/empty.c
index 11ec041..8d91e77 100644
--- a/Tests/RunCMake/CompileFeatures/empty.c
+++ b/Tests/RunCMake/CompileFeatures/empty.c
@@ -1,7 +1,7 @@
 #ifdef _WIN32
 __declspec(dllexport)
 #endif
-  int empty()
+  int empty(void)
 {
   return 0;
 }