Configure qt5.git with
Make sure you don't have any Preload.cmake file or anything else that would set an explicit prefix or enable developer-build feature (non-prefix build).
Upon the end of configuration, you get the following output
If you reconfigure with same args in the same folder again a second time, you get
The build turns into a non-prefix build, as evidenced by checking that QT_WILL_INSTALL is set to OFF in CMakeCache.txt.
The symptomatic flip is observed in the second cmake call, but the reason is in the first call.
With --trace-redirect --trace-expand at the first configure call, one can see that when QtBuildInternalsExtra.cmake is loaded during qtsvg configuration, CMAKE_INSTALL_PREFIX is reset from /usr/local to the qtbase build folder.
This only happens in a top-level build with more than 1 repo.
Doesn't seem to happen if an explicit -prefix or CMAKE_INSTALL_PREFIX is passed when configuring, but not sure, maybe it's just not observable in the output and something might still go wrong.