update CMake

[nim65s]

• use CMake to get M3T instead of git submodule
• use CMake to get pybind11 instead of pip
• use option() for kinect / realsense / gtest, to allow eg. cmake -DUSE_REALSENSE=ON or cmake -DUSE_REALSENSE=OFF
• remove CMAKE_CXX_STANDARD: we want at least 17, so don't require exactly 17. use cxx_std_17 instead, ref. https://cmake.org/cmake/help/latest/manual/cmake-compile-features.7.html#requiring-language-standards
• remove CMAKE_BUILD_TYPE to allow users specify themselves eg. cmake -DCMAKE_BUILD_TYPE=Debug or export CMAKE_BUILD_TYPE=RelWithDebInfo
• remove -fPIC, as it looks like it is not necessary.

A better solution would be to make CMake look for M3T & pybind11, and fetch those only if they are not found. It would allow downstream package managers to provide their own versions and avoid dynamic downloads at configure time.