GenEx: Fix TARGET_PROPERTY evaluation of transitive link properties

In commit bbba701899 (Link properties: must be transitive over private
dependency on static library, 2019-12-06, v3.17.0-rc1~323^2) and
commit af9d4f24ae (Link properties: must be transitive over private
dependency on static library, 2019-12-11, v3.17.0-rc1~305^2) we
neglected to implement CMP0099 NEW behavior for `TARGET_PROPERTY`
evaluation.  Add policy CMP0166 to fix this.