cmFindCommon: Revert accidental change

In commit c4373b33 (cmTarget: Make GetProperty() const, 2013-10-29)
we accidentally changed the cmFindCommon.cxx logic in a way that
looks like a local experiment leftover that went unnoticed due to
the size of the other changes in the commit.  Revert it.
Merge topic 'revert-accidental-changes' into next

13aea6d cmFindCommon: Revert accidental change