Fix force build for CTK

Replace the 'build depends on always run step' rule for CTK (which does
not work with at least Ninja) with a rule to remove the stamp for the
build step. This more reliably ensures that the build step for CTK is
always executed.
Merge pull request #458 from mwoehlke-kitware/fix-superbuild-ctk-force-build

Fix force build for CTK