At the moment PackageKit fails in JHBuild when QtCore is not found, so I had to forcefully disable Qt support (forcing --disable-qt when running configure).
It would be nice if Qt support was automatically enabled when the required packages are found, and disabled otherwise.
I think the logic was to not have "silent" enabling and disabling of large chucks of PackageKit like this. If you email the mailing list we can discuss with the guys maintaining the build scripts what the logic was, and if it can be changed. Thanks.