In a previous post, I had confused the two kinds of commands. However, I still don't understand why Code::Blocks insists on specifying
-Weverything even though I have only asked for -Wall. Can you shed light on that?
It is probably acivated project-wide (see See project -> build options -> select your project name (top left)).
Often, the project and target options are concatenated but you can limit that to target only if you want.
See project -> build options -> your llvm target -> policy: use target options only. This can be set individually for compile- link-flags etc.
Quite comfortable. Play with it.
If the unwanted option is still there, than it is configured systemwide. See reply by oBFusCATed to find its location.
If you want to use the wizard without updating your cb: You can start a project with gcc and later on switch the selected compiler to llvm/clang.
or even create your personal compiler-settings by copying and adapting one of the existing ones (see settings -> compiler) and use that.