Ensuring low-latency and highly reliable communication between vehicles is one of the goals of 5G. We present a 5G Vehicle-to-everything (V2X) wireless testbed based on flexible and re-configurable software defined radio that is designed for cooperative automated driving. The use-cases and communication requirements for cooperative automated driving are discussed to motivate the system design and technical enablers that can achieve the most stringent link-level communication requirements of cooperative autonomous driving. The key building blocks include a re-configurable RF front-end, optimized base-band processing on standard Intel CPUs and a custom-built high-power external RF subsystem. The technical enablers include a new OFDM-like waveform based on Pulse-shaping, a flexible and self-contained frame-structure design, GNSS-aided hybrid synchronization and low-latency scheduled multiple-access. We finally present some experimental results from lab measurements.