configure: rename cuda to ffnvcodec

Right now, if someone configures ffmpeg with for example --enable-nvenc they will
get an error message complaining about missing cuda.
This is very confusing and already has lead people into installing the CUDA SDK,
even though it's not what they need.

This will make it complain about ffnvcodec instead.

Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>