Do not use own warning-as-error logic, if standard CMake flag is in us.

If CMAKE_COMPILE_WARNING_AS_ERROR is set, ígnore DIY logic.
CMAKE_COMPILE_WARNING_AS_ERROR is available since CMake 3.24

-DCMAKE_COMPILE_WARNING_AS_ERROR=0 will also help to avoid currently
unconditional "warning is error" when building standalone or via
ExternalProject_Add

CMakeLists.txt

@@ -395,7 +395,7 @@ ENDIF()
 MESSAGE1(SYSTEM_LIBS "SYSTEM_LIBS ${SYSTEM_LIBS}")
 MARK_AS_ADVANCED(SYSTEM_LIBS)
 
-IF(NOT IS_SUBPROJECT)
+IF(NOT IS_SUBPROJECT AND (NOT DEFINED CMAKE_COMPILE_WARNING_AS_ERROR))
 IF ((NOT WIN32) AND (CMAKE_C_COMPILER_ID MATCHES "Clang" OR CMAKE_C_COMPILER_ID MATCHES "GNU"))
   SET(WARNING_AS_ERROR "-Werror")
 ELSEIF(CMAKE_C_COMPILER_ID MATCHES "MSVC")