Default CMake config leaves all assertions enabled

[gcp]

This is defaulted (on GCC-like platforms): https://github.com/glinscott/leela-chess/blob/master/CMakeLists.txt#L72

Which is missing NDEBUG: https://github.com/glinscott/leela-chess/blob/master/CMakeLists.txt#L68

Now, the optimization flags in the default config are lowered a bit too, so I'm not sure this is intentional or not, but in general one doesn't expect DEBUG stuff to be enabled in what is supposed to be a default or release build.

I suspect this is also what causes NNCache debug info to be spewed out, as it's protected by the same flag.

If it's agreed this was not intentional, I'll make a pull request.