Ah ha (so to speak)! In that case, @zhuhongfei needs to explain what it is he is doing from a MinGW build which requires Qt5cored.dll? Is it possible he is doing something only to do with MSVC?
@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.
Thanks J-P, that's exactly what I needed to know! I think I found an old example online that used the 1.x approach to styling, then tried to combine that with 2.x controls - and got myself in a tangle. I now understand how styling is meant to be applied in 2.x, and it's working just fine for me.