Configuration, CMake - the "custom" script isn't created in a build dir if it exists in an install dir

custom.sh/bat isn't created if CMake configuration process has found a file with the same name in a further install dir

also CMake config process updates custom.bat (if it exists) in the install dir by adding extra lines and it doesn't check whether these line are already added. The result of this action is a redundant code in the script file