CMakeLists: Define QT_USE_QSTRINGBUILDER for the Qt target

This is a compile definition introduced in Qt 4.8 for reducing the total
potential number of strings created when performing string
concatenation. This allows for less memory churn.

This can be read about here:
https://blog.qt.io/blog/2011/06/13/string-concatenation-with-qstringbuilder/

For a change that isn't source-compatible, we only had one occurrence
that actually need to have its type clarified, which is pretty good, as
far as transitioning goes.