Require C++11 through cmake

Summary:
This replaces the previous solution to require at least gcc 4.8 which
did not cover clang. As cmake has a proper way to check for 11, let's
use that.

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D4486