try_compile: Propagate CMP0126 to the generated test project

Set policy CMP0126 to the value used in the calling project.
It may affect toolchain file behavior.

Source/cmCoreTryCompile.cxx

@@ -570,6 +570,13 @@ int cmCoreTryCompile::TryCompileCode(std::vector<std::string> const& argv,
               *cmp0123 == "NEW"_s ? "NEW" : "OLD");
     }
 
+    /* Set cache/normal variable policy to match outer project.
+       It may affect toolchain files.  */
+    if (this->Makefile->GetPolicyStatus(cmPolicies::CMP0126) !=
+        cmPolicies::NEW) {
+      fprintf(fout, "cmake_policy(SET CMP0126 OLD)\n");
+    }
+
     std::string projectLangs;
     for (std::string const& li : testLangs) {
       projectLangs += " " + li;

Tests/RunCMake/ToolchainFile/CMP0126-NEW-stderr.txt

+^try_compile CMP0126='NEW' VAR='1'

Tests/RunCMake/ToolchainFile/CMP0126-NEW-toolchain.cmake

+include("${CMAKE_CURRENT_LIST_DIR}/CMP0126-toolchain.cmake")

Tests/RunCMake/ToolchainFile/CMP0126-NEW.cmake

+cmake_policy(SET CMP0126 NEW)
+enable_language(C)

Tests/RunCMake/ToolchainFile/CMP0126-OLD-stderr.txt

+^try_compile CMP0126='OLD' VAR='2'

Tests/RunCMake/ToolchainFile/CMP0126-OLD-toolchain.cmake

+include("${CMAKE_CURRENT_LIST_DIR}/CMP0126-toolchain.cmake")

Tests/RunCMake/ToolchainFile/CMP0126-OLD.cmake

+cmake_policy(SET CMP0126 OLD)
+enable_language(C)

Tests/RunCMake/ToolchainFile/CMP0126-WARN-stderr.txt

+^try_compile CMP0126='OLD' VAR='2'

Tests/RunCMake/ToolchainFile/CMP0126-WARN-toolchain.cmake

+include("${CMAKE_CURRENT_LIST_DIR}/CMP0126-toolchain.cmake")

Tests/RunCMake/ToolchainFile/CMP0126-WARN.cmake

+# leave CMP0126 unset
+enable_language(C)

Tests/RunCMake/ToolchainFile/CMP0126-toolchain.cmake

+get_property(_IN_TC GLOBAL PROPERTY IN_TRY_COMPILE)
+if(_IN_TC)
+  cmake_policy(GET CMP0126 cmp0126)
+  set(VAR 1)
+  set(VAR 2 CACHE STRING "")
+  message("try_compile CMP0126='${cmp0126}' VAR='${VAR}'")
+endif()

Tests/RunCMake/ToolchainFile/RunCMakeTest.cmake

@@ -11,6 +11,9 @@ run_cmake_toolchain(CheckLanguage)
 run_cmake_toolchain(FlagsInit)
 run_cmake_toolchain(LangVars)
 run_cmake_toolchain(LinkFlagsInit)
+run_cmake_toolchain(CMP0126-NEW)
+run_cmake_toolchain(CMP0126-OLD)
+run_cmake_toolchain(CMP0126-WARN)
 
 function(run_IncludeDirectories)
   run_cmake_toolchain(IncludeDirectories)