Show me one C/C++ library that competes with Theano or Torch7?

Google / Facebook and many other huge companies are using Theano and Torch7 in production, at scale. The ML industry has been continuously moving in this direction for years now.

On these optimized ML systems, only a tiny fraction of CPU time is spent outside of the GPU. The goal in many of these companies is to migrate all tasks that can be done on GPUs to GPUs, as soon as possible. It's far faster and more cost efficient.