configure should fail if xcb devel package is not installed on linux

If I try to build qt5 without having the pre-requisite libxcb-xfixes0-dev package installed, configure will still succeed, even though the resulting build is useless as all binaries abort on startup. Configure must fail with a meaningful error message if any prerequisites are missing, otherwise we're just wasting the user's time on compiling a build that cannot possibly do what the user wants.