fix build error on Ubuntu 16.04

[niosus]

There was a bunch of build errors on Ubuntu 16.04. I've fixed them, but did not test if the fixes break anything on Windows.

Short summary:

• fix missing QtConcurrent header. It is not present on Ubuntu.
• fix c++11 configuration not being read
• fix vector access. No takeAt method available. Replaced with generic at method. In case you don't need to check the boundaries, one can replace it with [index].