If one of the libraries (_DEBUG or _RELEASE) is not set, the value is
set to the value of the other one. FindQt4, from which the macro is
extracted, sets the values to XXX_LIBRARY_{DEBUG,RELEASE}-NOTFOUND
instead. In both cases the XXX_LIBRARY is correct, but using NOTFOUND
makes it easier to understand which one is missing.
Update Tests/CMakeOnly/SelectLibraryConfigurations with the new logic.