To build Koinos system contracts, you must first build and install the CDT.
Set the following env vars as was needed to build the CDT.
KOINOS_SDK_ROOT and KOINOS_WASI_SDK_ROOT.
Then run the following commands to do an out of source cmake build.
mkdir build
cd build
cmake -DCMAKE_TOOLCHAIN_FILE=${KOINOS_SDK_ROOT}/cmake/koinos-wasm-toolchain.cmake -DCMAKE_BUILD_TYPE=Release ..
make -j
.wasm binaries are in your build directory and are ready to be uploaded to Koinos.