[REG 6.1.0 -> 6.2] developer-build needs to be installed

1. Check out qt5 and its submodule qtbase.
   I have da632b95d2289d1a963233fc8106f8e988b8f1ff and 75d8623752e67a39e2ee04e5233dc502e63a17f1 respectively.
2. Configure Qt for a developer build.
   My command line:

   /tmp/qt-bisect/qt5/configure -developer-build -opensource -confirm-license -nomake examples -nomake tests
   

   When configure finishes, it will write:

   Once everything is built, you must run 'cmake --install .'
   Qt will be installed into '/usr/local'
   

   A developer build didn't need installing in years.

Qt 6.1.0 will write the following instead:

Once everything is built, Qt is installed. You should NOT run 'cmake --install .'