Why is there `No CMAKE_CXX_COMPILER could be found.` and how to solve?

I just installed cmake but I'm getting compiler not found error. In trying to build https://gitlab.com/interception/linux/tools on a new Kubuntu installation, running cmake .. from the tools/build directory returns the error:

CMake Error at CMakeLists.txt:3 (project):
  No CMAKE_CXX_COMPILER could be found.

  Tell CMake where to find the compiler by setting either the environment
  variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
  to the compiler, or to the compiler name if it is in the PATH.

What's wrong? I assumed cmake would be equipped with its compiler, but maybe it needs to be configured before it can be used???