VTK-m now provides a default and a list of types for CMAKE_BUILD_TYPE.
CMake has several default build types, but if nothing is specified when configuring your project it defaults to an empty string and no optimization flags are used.
It will now default to using a debug build if the source directory is a git clone, or a release build if not. Additionally when using ccmake or cmake-gui this will provide a nice list of possible options for CMAKE_BUILD_TYPE.