@aha_1980 Thank you so much for your ideas. Both problems solved.
After reconfiguring aptitude stuff to use "stable" instead of hard-coded "jessie" and half-a-day-long upgrades and cleaning the mess I really got not only higher version of Qt, but as well QParallelPort working as planned. Definitely worth the effort (even if I did not expect the upgrade to take so much time.)
My mistake. I missed this single character overload to append(). Thank you for pointing me there. Now I got completely rid of that development/release conditionals.
Case closed, thanks to all who assisted.