DEPENDS_EXPLICIT_ONLY for all add_custom_command

This MR added "DEPENDS_EXPLICIT_ONLY " option to “add_custom_command”, would it be helpful if there is a way to add it by default for all add_custom_command usages, probably a something like CMake variable like “CMAKE_ADD_CUSTOM_COMMAND_DEPENDS_EXPLICIT_ONLY” ?

I’ve been thinking this should help to reduce coding style, as well as be a way to set this option for custom commands used in CMake exported macros, like ones in (FindBISON, FindFLEX, FindProtobuf, …).

Adding a variable to opt-in for all add_custom_command calls sounds reasonable.