Я пытаюсь собрать Qt5 на Ubuntu 20.04:
$ sudo apt-get build-dep qt5-default
$ sudo apt-get install libxcb-xinerama0-dev
$ sudo apt-get install '^libxcb.*-dev' libx11-xcb-dev libglu1-mesa-dev \
libxrender-dev libxi-dev libxkbcommon-dev libxkbcommon-x11-dev
$ cd ~
$ mkdir qt
$ cd qt
$ git clone https://code.qt.io/qt/qt5.git
$ cd qt5
$ git checkout 5.15
$ perl init-repository
$ cd ..
$ mkdir qt5-build
$ cd qt5-build
$ ../qt5/configure -developer-build -opensource -nomake tests
[...]
Creating qmake...
......................................................
.........................................Done.
Could not find qmake spec ''.
Error processing project file: /home/hakon/qt/qt5/qt.pro
Я также проверил значение QMAKESPEC:
$ qmake -query QMAKESPEC
**Unknown**