VS 6: Tell BuildDepends test to tolerate ninjadep failure

The VS 6 IDE does not want to recompile a particular source after
a particular header it includes is modified, even by hand.  For
now just silence the failure and document it with a comment.
Merge topic 'ninja_bad_cmcldeps_paths' into next

d12459f VS 6: Tell BuildDepends test to tolerate ninjadep failure