Fix build and autopkgtest with Qt 5.15

Dmitry Shachnevrequested to merge
mitya57/qTox:qt5.15 into master
  • Backported upstream changes from three commits to fix new errors and warnings from Qt 5.15.
  • Disabled CMake warnings about use of deprecated function (qt5_make_output_file). This is fixed upstream, but the commit that fixes it is not easily backportable.

Together, these two changes fix #973540.

@yangfl-guest Please let me know if you can review these changes. This is blocking the transition, so if you cannot then I will upload an NMU.

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