cmTarget: Fix debug report for interface-set compatibility types.
If the dependent target sets the property to boolean false, ensure that that appears in the debug report. Previously, the report output contained whether the property was consistent among dependencies, displaying 'TRUE', instead of the content of the property, which may be 'FALSE'. Return a std::pair from the consistentProperty method. This makes it possible to make the return value for string types easier to reason about. The return value of consistentProperty was previously set to an empty static string to emulate a 'true' value for the caller in commit 816b4a8a (cmTarget: Make consistentProperty return consistent content., 2013-10-22). The pair makes the consistency result properly typed.