Building with qmake did not work on macOS or at least not on all macOS
systems. This can be fixed by not compiling the header-only json library
into an empty static library - which is not needed anyway.
See issue #2014.
Replace Qt's own JSON library functions by Niels Lohmann's JSON library
in the Export JSON dialog. This was necessary because for very large
JSON objects Qt's library functions generated incomplete JSON exports.
See issue #1789.