Its funny, modules such as these even though they are not mentioned as dependencies, they often "go together", I myself assume if one is installed the other is, but it seems that is not the case and you must specifically ask for each of these per each version to have them installed and available: webkit/engine->webkit/enginewidgets->positioning, all in the maintenence tool but hiding under the <version>->extensions group.
Also note that QT does minimal or no set-up of your environment when it comes to runtime libraries. You will find more often than not that you have to either put these libraries into your system's library install folders OR (what I prefer to do) tell your library config tool about the additional locations.
Example:
For Linux 64-bit debian distributions: /etc/ld.so.conf.d/x86_64-linux-gnu.conf holds locations of all 64-bit libraries. Note there are some naming and directory structure requirements for complex hardware architecture requirements such as sub-archetecture (for example, under x86, we have emt64, mmx, etc) please see your system's ld.so(8) manpage for your system's specific understanding of that (yes the manpages can and will differ from system to system).