cmstd: Modernize CMake system headers

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

Utilities/std/cm/iterator

@@ -0,0 +1,78 @@
+// -*-c++-*-
+// vim: set ft=cpp:
+
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cm_iterator
+#define cm_iterator
+
+#include <iterator> // IWYU pragma: export
+
+namespace cm {
+
+#if __cplusplus >= 201402L || defined(_MSVC_LANG) && _MSVC_LANG >= 201402L
+using std::make_reverse_iterator;
+
+using std::cbegin;
+using std::cend;
+#else
+template <class Iter>
+std::reverse_iterator<Iter> make_reverse_iterator(Iter it)
+{
+  return std::reverse_iterator<Iter>(it);
+}
+
+// std::c{begin,end} backport from C++14
+template <class C>
+#  if defined(_MSC_VER) && _MSC_VER < 1900
+auto cbegin(C const& c)
+#  else
+constexpr auto cbegin(C const& c) noexcept(noexcept(std::begin(c)))
+#  endif
+  -> decltype(std::begin(c))
+{
+  return std::begin(c);
+}
+
+template <class C>
+#  if defined(_MSC_VER) && _MSC_VER < 1900
+auto cend(C const& c)
+#  else
+constexpr auto cend(C const& c) noexcept(noexcept(std::end(c)))
+#  endif
+  -> decltype(std::end(c))
+{
+  return std::end(c);
+}
+#endif
+
+#if __cplusplus >= 201703L || defined(_MSVC_LANG) && _MSVC_LANG >= 201703L
+using std::size;
+#else
+
+// std::size backport from C++17.
+template <class C>
+#  if !defined(_MSC_VER) || _MSC_VER >= 1900
+constexpr
+#  endif
+  auto
+  size(C const& c) -> decltype(c.size())
+{
+  return c.size();
+}
+
+template <typename T, size_t N>
+#  if !defined(_MSC_VER) || _MSC_VER >= 1900
+constexpr
+#  endif
+  std::size_t
+  size(const T (&)[N]) throw()
+{
+  return N;
+}
+
+#endif
+
+} // namespace cm
+
+#endif