In commit ff6c4013 (cmTarget: Add interface for compatible numeric
properties, 2013-10-22, v3.0.0-rc1~460^2) the condition

    dagCheckerParent && !dagCheckerParent->EvaluatingLinkLibraries()

was written that way to avoid a nullptr dereference, but is actually
meant to say "is not evaluating link libraries".  That can also be true
when there is no `dagCheckerParent`, such as when evaluating a generator
expression outside of usage requirements, e.g., for `add_custom_target`.
The original commit tried to account for that by duplicating the
implementation in another code path, but that did not work in all cases.

Fix the condition, remove the duplication, and enable tests for the
now-working cases.