Tests/CustomCommand/check_mark.cmake - third_party/github.com/Kitware/CMake - Git at Google

 if(EXISTS "${MARK_FILE}")
   message(FATAL_ERROR "Custom command run more than once!")
 else()
   file(WRITE "${MARK_FILE}" "check for running custom command twice\n")
 endif()
	if(EXISTS "${MARK_FILE}")
	message(FATAL_ERROR "Custom command run more than once!")
	else()
	file(WRITE "${MARK_FILE}" "check for running custom command twice\n")
	endif()