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

bootstrap

@@ -453,7 +453,6 @@ CMAKE_CXX_SOURCES="\
   cmake  \
   cmakemain \
   cmcmd  \
-  cm_string_view \
 "
 
 if ${cmake_system_mingw}; then
@@ -464,6 +463,17 @@ if ${cmake_system_mingw}; then
   "
 fi
 
+CMAKE_STD_CXX_HEADERS="\
+  memory \
+  optional \
+  shared_mutex \
+  string_view \
+  utility \
+"
+CMAKE_STD_CXX_SOURCES="\
+  string_view \
+"
+
 LexerParser_CXX_SOURCES="\
   cmCommandArgumentLexer \
   cmCommandArgumentParser \
@@ -1346,6 +1356,8 @@ cmake_compiler_settings_comment="/*
  *
  * Sources:
  * ${CMAKE_CXX_SOURCES} ${CMAKE_C_SOURCES}
+ * STD Sources:
+ * ${CMAKE_STD_CXX_HEADERS} ${CMAKE_STD_CXX_SOURCES}
  * LexerParser Sources:
  * ${LexerParser_CXX_SOURCES} ${LexerParser_C_SOURCES}
  * kwSys Sources:
@@ -1412,9 +1424,12 @@ done
 cmake_generate_file "${cmake_bootstrap_dir}/cmThirdParty.h" ""
 
 # Generate Makefile
-dep="cmConfigure.h cmsys/*.hxx cmsys/*.h `cmake_escape \"${cmake_source_dir}\"`/Source/*.h"
+dep="cmConfigure.h cmsys/*.hxx cmsys/*.h `cmake_escape \"${cmake_source_dir}\"`/Source/*.hxx `cmake_escape \"${cmake_source_dir}\"`/Source/*.h"
+for h in ${CMAKE_STD_CXX_HEADERS}; do
+  dep="${dep} `cmake_escape \"${cmake_source_dir}\"`/Utilities/std/cm/${h}"
+done
 objs=""
-for a in ${CMAKE_CXX_SOURCES} ${CMAKE_C_SOURCES} ${LexerParser_CXX_SOURCES} ${LexerParser_C_SOURCES} ${KWSYS_CXX_SOURCES} ${KWSYS_C_SOURCES}; do
+for a in ${CMAKE_CXX_SOURCES} ${CMAKE_C_SOURCES} ${CMAKE_STD_CXX_SOURCES} ${LexerParser_CXX_SOURCES} ${LexerParser_C_SOURCES} ${KWSYS_CXX_SOURCES} ${KWSYS_C_SOURCES}; do
   objs="${objs} ${a}.o"
 done
 for a in ${LIBUV_C_SOURCES}; do
@@ -1498,6 +1513,7 @@ cmake_cxx_flags="${cmake_cxx_flags} \
   -I`cmake_escape \"${cmake_bootstrap_dir}\"` \
   -I`cmake_escape \"${cmake_source_dir}/Source\"` \
   -I`cmake_escape \"${cmake_source_dir}/Source/LexerParser\"` \
+  -I`cmake_escape \"${cmake_source_dir}/Utilities/std\"` \
   -I`cmake_escape \"${cmake_source_dir}/Utilities\"`"
 echo "cmake: ${objs}" > "${cmake_bootstrap_dir}/Makefile"
 echo "	${cmake_cxx_compiler} ${cmake_ld_flags} ${cmake_cxx_flags} ${objs} ${libs} -o cmake" >> "${cmake_bootstrap_dir}/Makefile"
@@ -1512,6 +1528,12 @@ for a in ${CMAKE_C_SOURCES}; do
   echo "${a}.o : ${src} ${dep}" >> "${cmake_bootstrap_dir}/Makefile"
   echo "	${cmake_c_compiler} ${cmake_c_flags} -c ${src} -o ${a}.o" >> "${cmake_bootstrap_dir}/Makefile"
 done
+for a in ${CMAKE_STD_CXX_SOURCES}; do
+  src=`cmake_escape "${cmake_source_dir}/Utilities/std/cm/bits/${a}.cxx"`
+  src_flags=`eval echo \\${cmake_cxx_flags_\${a}}`
+  echo "${a}.o : ${src} ${dep}" >> "${cmake_bootstrap_dir}/Makefile"
+  echo "	${cmake_cxx_compiler} ${cmake_cxx_flags} ${src_flags} -c ${src} -o ${a}.o" >> "${cmake_bootstrap_dir}/Makefile"
+done
 for a in ${LexerParser_CXX_SOURCES}; do
   src=`cmake_escape "${cmake_source_dir}/Source/LexerParser/${a}.cxx"`
   src_flags=`eval echo \\${cmake_cxx_flags_\${a}}`