Fix compiler detection and let Qt decide if examples should be built

This fixes the build on OS X if the path to clang is not cached,
and only builds examples if Qt was configured to do so.
This has already been fixed in dev.

Change-Id: Ia2d17f38f30e832425f6e1cd014b46a6c6ae78ec
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>