When re-configuring a build, it is possible to do so in an unclean way on a stale build folder.

The average person initiating such a build does not know when a stale build is safe and when not. Time is wasted for the builder when a broken build silently succeeds and then leads to a nonsensical error later on. Just to give the last two examples of the last few days: (#31922 (comment), #31933 (comment), ...)

The benefit of an unclean build is unclear, given that it merely saves a few seconds of a build that normally happens in the background anyway, at the risk of wasting minutes or hours if something is overlooked.

Thus, a build configure should be rejected if the build directory exists.

(Obviously, re-builds on a build directory that do not require a re-configure are not affected)