Port to std::chrono but allow fallback to boost::chrono if unavailable

Port away from boost::chrono to std::chrono. Since std::chrono support
requires C++ 2011 mode on the compiler and since mixed-type arithmetic
using doubles is potentially iffy on certain implementations, a fallback
mechanism to select boost::chrono is provided.

Correct chrono to use is auto-detected, feature-detection implementation
copied from KActivities (thanks to Ivan Cukic for doing all the hard
work of figuring out cmake 'magic'!).

REVIEW: 124615