Linux distributions and security aware users do want to avoid the use of Third-Party source code in Qt as much as possible. However, configure does in most cases use the third-party sources in src/3rdparty as an automatic fallback when a matching system library is not found:

Qt comes with bundled copies of some 3rd party libraries. These are used
by default if auto-detection of the respective system library fails.

We should give users an option to inverse this logic: Make disabling the whole feature the default (if a system library is not found), but provide means to opt-in especially:

# Disable all -qt-xxx features, except 
configure -no-bundled-thirdparty -qt-harfbuzz