Fix frequent rebuild of options target (#7450)

Fix frequent rebuild of options target (#7450)

The mkoptions.py script only updates its output files if their content would actually change. This avoid a full rebuild on every run, and makes sure that only parts that actually change are rebuild.
Unfortunately this interacts badly with how cmake/make/... do inter-target dependency tracking.
This PR adds a stamp file options.stamp that is always updated by mkoptions.py and used by cmake as main output.