1. Clone Qt:

   git clone -b 6.2 git://code.qt.io/qt/qt5.git
   

2. Initialize the repository:

   cd qt5/
   perl init-repository --codereview-username rlohning --module-subset=qtbase,qttools,qttranslations
   

   I guess you don't need to use my name, but who knows...

3. Change to a build directory:

   cd ..
   mkdir build
   cd build
   

4. Configure Qt:

   /tmp/assi/qt5/configure -release -prefix $PWD/qtbase -opensource -confirm-license
   

   Using the actual location of your clone.

5. Build Qt:

   cmake --build . -j6
   

6. After the build succeeded and without changing anything, try building Qt again:

   cmake --build . -j6
   

   You'll get a couple of errors like:

   FAILED: qttranslations/translations/CMakeFiles/updateqm-qtmultimedia_de.ts qtbase/translations/qtmultimedia_de.qm /tmp/assi/build/qttranslations/translations/CMakeFiles/updateqm-qtmultimedia_de.ts /tmp/assi/build/qtbase/translations/qtmultimedia_de.qm
   cd /tmp/assi/build/qttranslations/translations && /home/qtrob/qt-cmake/Tools/CMake/bin/cmake -E make_directory /tmp/assi/build/qtbase/./translations && /tmp/assi/build/qtbase/bin/lrelease /tmp/assi/qt5/qttranslations/translations/qtmultimedia_de.ts -qm /tmp/assi/build/qtbase/./translations/qtmultimedia_de.qm
   Updating '/tmp/assi/build/qtbase/./translations/qtmultimedia_de.qm'...
   Bus error (core dumped)
   

The second build should be a no-op. At the very least, it should not fail.