This is The Right Way to detect libclang. Using a Probe will also
slightly improve performance in qbs 1.6 due to the result caching.
Change-Id: I063a8d108d02b620dda2df75dd8c014c84f27ec7
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>