Remove temporary allocations by extending the lifetime of the retval.

See also Herb Sutter's article on the "most important const":
http://herbsutter.com/2008/01/01/gotw-88-a-candidate-for-the-most-important-const/

When running the CMake daemon on the KDevelop build dir, this removes
some hundreds of thousands of temporary allocations.

This hotspot was found with heaptrack.