NinJa_version

[wangmiaowei]

While, When I build the pointnet++, I get the ninja -version wrong:UserWarning: Attempted to use ninja as the BuildExtension backend but we could not find ninja.. Falling back to using the slow distutils backend.
warnings.warn(msg.format('we could not find ninja.'))
copying build/lib.linux-x86_64-3.6/_ext.cpython-36m-x86_64-linux-gnu.so ->
My ninja --version output is ：1.10.2

Could you help me solve this problem?