CMAKE_COMPILER_IS_GNU*: improve documentation
These variables received a versionadded
directive in 496ec603, but that is wrong. The script that added the directives probably picked up the changes added in !103 (merged).
The real origin of these variables is f5d95fb0, which predates 2.4.0 even.
I also think the documentation should have a stronger suggestion for the alternative already suggested Maybe deprecation should also be considered, but the stronger language in the docs might make that unnecessary.
I haven't made a MR for this because of the last point.
Edited by friendlyanon