May I ask why it shows build.xml does not exist when I build.
Check if you have a build.xml file in the root directory where you have the cesium source-code.
If you dont have it.
Try to download the complete source-code once again.
If you have it.
You probably are running the command wrong.
try running " ./Tools/apache-ant-1.8.2/bin/ant combine runServer " without the quotes from the root cesium directory itself.
Hope it helps!