Set the number of threads in a CMake build

According to the Release Notes, with CMake 3.12 it can be done cross-platform:

The cmake(1) Build Tool Mode (cmake –build) gained --parallel [<jobs>] and -j [<jobs>] options to specify a parallel build level. They map to corresponding options of the native build tool.

You can pass arbitrary arguments to the native build tool with --. Everything after -- will be passed to the build tool. To pass -j 3 in your example, just use

cmake --build . --config Release -- -j 3

Documentation: https://cmake.org/cmake/help/v3.5/manual/cmake.1.html

You could also use Ninja as a build tool, it uses automatically an appropriate number of threads. Or you can modify the make command by defining CMAKE_MAKE_PROGRAM="make -j 3. But this is a rather hacky workaround.