BlueZ-check during build is too unobtrusive

When building Qt for Linux the BlueZ header files are required to enable the bluetooth functionality (libbluetooth-dev package on Debian). But this precondition is not documented (or the requirement is very well hidden in the documentation).
Furthermore the check is not done during configure but during make and produces just a simple one-line message "Unsupported bluetooth platform, will not build a working QtBluetooth library" between of 25 MB of other compiler output.

It's very hard to spot this problem if you just get a runtime warning that Qt does not support Bluetooth on your platform. So the check should be done during configure with a noticeable warning in the summary.