Add qtwebengine configure line saying if it will be skipped

We have a number of mixed conditions that can cause qtwebengine to be skipped. Unfortunately many of them only trigger when we get to building qtwebengine, and are not listed in the configure sumary.

We should try to figure out if we will skip qtwebengine earlier and tell the user in the configure overview.

This will likely require some rewriting of the platform checks to be performable during configure.