I know that you can add -j1 flag to make, but how can you do it in QMake?
Long time ago there was a variable MAKEFLAGS but now there’re hundreds of different variables instead:
https://doc.qt.io/qt-5/qmake-variable-reference.html

I tried to add this flag to QMAKE_LFLAGS, to QMAKE_CXXFLAGS and so on, it didn’t help, I still got this line in compile output

15:23:29: Starting: “/usr/bin/make” -j2