-std=gnu++11 vs -std=c++11

[springmeyer]

Cmake, for some reason, defaults to -std=gnu++11 instead of -std=c++11. We want to specify the latter rather than the former. This ticket stands to track doing this in a clean and concise way.

It looks as simple as doing set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11") per how OSRM does this: https://github.com/Project-OSRM/osrm-backend/blob/c718f140fa46d1fa0ee432289152c4328e31453a/CMakeLists.txt#L383-L386. More background from @daniel-j-h at mapbox/spatial-algorithms#9 (comment).