For now, compiler extensions are no longer activated on CMake sources.
However these extensions are still used for various third parties.
This MR is a partial answer to the issue #20454.
Provide a standardized way to handle the C++ "standard" headers
customized to be used with current CMake C++ standard constraints.
Offer under directory `cm` headers which can be used as direct
replacements of the standard ones. For example:
#include <cm/string_view>
can be used safely for CMake development in place of the `<string_view>`
standard header.
Fixes: #19491
Marc Chevrier