Every policy's documentation has a paragraph on what version of CMake
introduced it, how to set the policy, and whether CMake warns if the
policy is not set. The wording of this paragraph has diverged across
policies over time. Factor the paragraph out into a standard advice
document included by every policy.
This was intended to be part of the initial MR (!6177), but accidentally went
missing when debugging nightly failures on less common systems. Noticed during
!6711 review as the comment about this behaviour didn't match the code.
Documentation for CMP0128 is updated to remove a false case and note the two
cases related to this.
Fixes#22224.
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.
Alex Turbov