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.