Commit a72e241200 (Check*: Restore more toleration of `;-W...` in
CMAKE_REQUIRED_FLAGS, 2026-07-21) updated the Check* modules to share
required-flag handling without adding coverage for those paths.
Exercise each probe with required definitions followed by a warning
option.
Without the compatibility handling, CMake rejects the warning option.
Without the required definitions, all seven checks fail to find their
header, functions, prototype, or type.
Issue: #28001
