configure does not error out with -xcb when deps are not satisfied

While testing https://codereview.qt-project.org/#/c/223880/ I found that configure ignores "-xcb" (explicit request for xcb plugin). When deps are not met we don't get an error like:

"ERROR: Feature 'xcb' was enabled, but the pre-condition 'libs.xcb' failed."

Build completes successfully, but XCB plugin obviously was not built because of the missing deps at configure time. This probably affects other platforms as well.

Also printing in "QPA backends" section could be improved. In the broken use case from the above we don't get a line for "X11: yes/no" at all.