Restore support for backslashes in initial language-wide flags

Refactoring in commit bdc40742 (CMakeDetermineCompilerId: Test without
COMPILER_ID_FLAGS if REQUIRE_SUCCESS, 2021-02-27, v3.20.0-rc3~6^2) added
an extra macro layer through which flag strings are passed.  That caused
an extra level of argument re-parsing, and broke flags with backslashes.
Pass flags to the helper macro through variable names instead.

Fixes: #22041

Modules/CMakeDetermineCompilerId.cmake

 # Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
 # file Copyright.txt or https://cmake.org/licensing for details.
 
-macro(__determine_compiler_id_test testflags_in userflags)
-  separate_arguments(testflags UNIX_COMMAND "${testflags_in}")
-  CMAKE_DETERMINE_COMPILER_ID_BUILD("${lang}" "${testflags}" "${userflags}" "${src}")
+macro(__determine_compiler_id_test testflags_var userflags_var)
+  separate_arguments(testflags UNIX_COMMAND "${${testflags_var}}")
+  CMAKE_DETERMINE_COMPILER_ID_BUILD("${lang}" "${testflags}" "${${userflags_var}}" "${src}")
   CMAKE_DETERMINE_COMPILER_ID_MATCH_VENDOR("${lang}" "${COMPILER_${lang}_PRODUCED_OUTPUT}")
 
   if(NOT CMAKE_${lang}_COMPILER_ID)
@@ -44,7 +44,8 @@ function(CMAKE_DETERMINE_COMPILER_ID lang flagvar src)
     endif()
 
     foreach(userflags "${CMAKE_${lang}_COMPILER_ID_FLAGS_LIST}" "")
-      __determine_compiler_id_test("${CMAKE_${lang}_COMPILER_ID_TEST_FLAGS_FIRST}" "${userflags}")
+      set(testflags "${CMAKE_${lang}_COMPILER_ID_TEST_FLAGS_FIRST}")
+      __determine_compiler_id_test(testflags userflags)
       if(CMAKE_${lang}_COMPILER_ID)
         break()
       endif()
@@ -55,7 +56,7 @@ function(CMAKE_DETERMINE_COMPILER_ID lang flagvar src)
     # of helper flags.  Stop when the compiler is identified.
     foreach(userflags "${CMAKE_${lang}_COMPILER_ID_FLAGS_LIST}" "")
       foreach(testflags ${CMAKE_${lang}_COMPILER_ID_TEST_FLAGS_FIRST} "" ${CMAKE_${lang}_COMPILER_ID_TEST_FLAGS})
-        __determine_compiler_id_test("${testflags}" "${userflags}")
+        __determine_compiler_id_test(testflags userflags)
         if(CMAKE_${lang}_COMPILER_ID)
           break()
         endif()

Tests/RunCMake/CMakeLists.txt

@@ -243,6 +243,7 @@ add_RunCMake_test(FileAPI -DPYTHON_EXECUTABLE=${PYTHON_EXECUTABLE}
 add_RunCMake_test(FindBoost)
 add_RunCMake_test(FindLua)
 add_RunCMake_test(FindOpenGL)
+add_RunCMake_test(InitialFlags)
 if(CMake_TEST_FindOpenSSL)
   add_RunCMake_test(FindOpenSSL)
 endif()

Tests/RunCMake/InitialFlags/C-stdout.txt

+CMAKE_C_FLAGS='[^']*-Denv="a\\b"[^']+-Dvar="b\\c"[^']*'

Tests/RunCMake/InitialFlags/C.cmake

+set(CMAKE_C_FLAGS_INIT [[-Dvar="b\c"]])
+enable_language(C)
+message(STATUS "CMAKE_C_FLAGS='${CMAKE_C_FLAGS}'")

Tests/RunCMake/InitialFlags/CMakeLists.txt

+cmake_minimum_required(VERSION 3.20)
+project(${RunCMake_TEST} NONE)
+include(${RunCMake_TEST}.cmake)

Tests/RunCMake/InitialFlags/CXX-stdout.txt

+CMAKE_CXX_FLAGS='[^']*-Denv="a\\b"[^']+-Dvar="b\\c"[^']*'

Tests/RunCMake/InitialFlags/CXX.cmake

+set(CMAKE_CXX_FLAGS_INIT [[-Dvar="b\c"]])
+enable_language(CXX)
+message(STATUS "CMAKE_CXX_FLAGS='${CMAKE_CXX_FLAGS}'")

Tests/RunCMake/InitialFlags/RunCMakeTest.cmake

+include(RunCMake)
+
+set(ENV{CFLAGS} "$ENV{CFLAGS} -Denv=\"a\\b\"")
+run_cmake(C)
+
+set(ENV{CXXFLAGS} "$ENV{CXXFLAGS} -Denv=\"a\\b\"")
+run_cmake(CXX)