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

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 https://github.com/mapbox/spatial-algorithms/issues/9#issuecomment-298584856.

Read the repository's CMake configuration first, then compare it with the referenced OSRM CMakeLists.txt example. Confirm the cleanest way to configure the project for -std=c++11 rather than -std=gnu++11, and verify that generated build flags use the requested standard.

Written by the indexing model from the issue text.