It looks like the build process was broken with this commit. They'll likely fix it soon, but for now you can do

cd cpp-ethereum
git checkout 5669c209f4f8dbb7dc3f744e8e2a9c92f1104ca3

and then go ahead with the rest of the build process

It looks like the build process was broken with this commit. They'll likely fix it soon, but for now you can do

cd cpp-ethereum
git checkout 5669c209f4f8dbb7dc3f744e8e2a9c92f1104ca3

and then go ahead with the rest of the build process

You can also open an issue here to bring this to the developer's attention. Please include detailed specs for your machine (OS version, any error logs, etc.)