Try something like this:
rm -rf root_src root_build root_install
git clone --branch v6-26-00-patches --depth=1 https://github.com/root-project/root.git root_src
mkdir root_build
cd root_build
cmake -Dall="ON" -DCMAKE_INSTALL_PREFIX=../root_install ../root_src
cmake --build . -- install -j4