Rename {_ => }CMAKE_INCLUDE_SYSTEM_FLAG_${lang}_WARNING

Although not documented publicly, projects may need to override
this variable.  Remove the private `_` prefix from its name.

Issue: #25103

Modules/Compiler/IntelLLVM.cmake

@@ -35,7 +35,7 @@ if(CMAKE_HOST_WIN32)
       if(CMAKE_${lang}_COMPILER_VERSION VERSION_GREATER_EQUAL "2021.4")
         set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang} "-external:I")
         if(CMAKE_${lang}_COMPILER_VERSION VERSION_GREATER_EQUAL "2022.2")
-          set(_CMAKE_INCLUDE_SYSTEM_FLAG_${lang}_WARNING "-external:W0 ")
+          set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang}_WARNING "-external:W0 ")
         endif()
       endif()
     endif()

Modules/Compiler/MSVC.cmake

@@ -20,6 +20,6 @@ macro(__compiler_msvc lang)
   # The `/external:I` flag was made non-experimental in 19.29.30036.3.
   if (CMAKE_${lang}_COMPILER_VERSION VERSION_GREATER_EQUAL 19.29.30036.3)
     set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang} "-external:I")
-    set(_CMAKE_INCLUDE_SYSTEM_FLAG_${lang}_WARNING "-external:W0 ")
+    set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang}_WARNING "-external:W0 ")
   endif ()
 endmacro()

Source/cmLocalGenerator.cxx

@@ -903,7 +903,7 @@ std::string cmLocalGenerator::GetIncludeFlags(
     sysIncludeFlag = this->Makefile->GetDefinition(
       cmStrCat("CMAKE_INCLUDE_SYSTEM_FLAG_", lang));
     sysIncludeFlagWarning = this->Makefile->GetDefinition(
-      cmStrCat("_CMAKE_INCLUDE_SYSTEM_FLAG_", lang, "_WARNING"));
+      cmStrCat("CMAKE_INCLUDE_SYSTEM_FLAG_", lang, "_WARNING"));
   }
 
   cmValue fwSearchFlag = this->Makefile->GetDefinition(

Source/cmVisualStudio10TargetGenerator.cxx

@@ -3449,7 +3449,7 @@ bool cmVisualStudio10TargetGenerator::ComputeClOptions(
 
       if (gotOneSys) {
         if (auto sysIncludeFlagWarning = this->Makefile->GetDefinition(
-              cmStrCat("_CMAKE_INCLUDE_SYSTEM_FLAG_", this->LangForClCompile,
+              cmStrCat("CMAKE_INCLUDE_SYSTEM_FLAG_", this->LangForClCompile,
                        "_WARNING"))) {
           flags = cmStrCat(flags, ' ', *sysIncludeFlagWarning);
         }