Fix issue where loading profiles with yaml-cpp 0.5.x would crash.

There seemed to be a bug when using yaml-cpp 0.5.x. At some point, the code was returning a reference to a temporary object, causing the yaml parser to crash.

This was noticed by the Krita maintainers when using the default internal Raw profile.

Along with this fix, I have improved the CMake code for detecting the system yaml version, as well as adding a check for the relevant boost headers (which yaml-cpp 0.5.x+ requires).