[Qt WebEngine] Make it easier to re-configure after installing missing libraries for qpa-xcb support

If some dependency for building Qt WebEngine is not found, the configure script prints a helpful warning, e.g.

WARNING: QtWebEngine won't be built. Python3 html5lib is missing.

Usually, installing the missing library and re-running the configure script would progress things further, and the previous failure would now pass...

...except for the libraries required for qpa-xcb support. The configure script does print a list of libraries that are found/not found, and installing one missing library and re-configuring does update the list. However, after we install everything, we would still get this cryptic message:

WARNING: QtWebEngine won't be built. Could not find all necessary libraries for qpa-xcb support. X11:YES LIBDRM:YES XCOMPOSITE:YES XCURSOR:YES XRANDR:YES XI:YES XPROTO:YES XSHMFENCE:YES XTST:YES

To progress further, it seems we must wipe the build folder clean and re-configure from scratch.

Suggestion
It would be good if the configure script could pick up the newly-installed libraries and continue where it left off.