cmake : bump to 3.12 (#768)

cmake : bump to 3.12 (#768)

This CMakeLists.txt file uses Cmake command `add_compile_definitions()`.

That command did not exist until CMake 3.12.

Reference documentation:
https://cmake.org/cmake/help/latest/command/add_compile_definitions.html

I see a failure in CMake 3.10 with existing CMakeLists.txt