Modernize CMake system headers
The goal of this MR is to provide a new and standardize way to handle the C++ "standard" headers customized to be used with current CMake C++ standard constraints.
The rule is to offer, under directory
cm, headers which can be used as direct replacement of the standard ones.
can be used safely for CMake developments as replacement of
<string_view> standard header.
Fixes: #19491 (closed)