I expect that the root cause is missing libffi(-dev[el]) on your system, preventing _ctypes from building, so your quick fix would be to install that and try again.

However, not having _ctypes shouldn't cause installation to fail.  Donald, is ctypes a hard requirement for pip to install itself?  If so, ensurepip as called by `make install` should either gracefully handle a lack of ctypes or be disabled when ctypes is not available.