Qt5 CI: add build configuration using LSB SDK

QTBUG-16385 wants to make Qt build nicely for LSB 4.

As the majority of developers will not use an LSB SDK for development, it is likely that, once the aforementioned bug is fixed, it will be repeatedly broken. We should add an LSB continuous integration configuration to block changes which would break the build.

It's unclear what is the most appropriate version of the LSB SDK - comments on QTBUG-16385 seem to indicate at least 4.1 will be necessary.

This task covers compilation only. Running the Qt test suite in an LSB-compliant environment is a separate chunk of work.