Understand C++ binding with OpenCV.

[k0zmo]

Most likely - when the Nim's manual was created GCC's ld.bfd that everyone used 
was built as such that `--as-needed` flag wasn't a default option. Since then 
most of the Linux distributions do package ld.bfd with `--as-needed` flag on by 
default. For dynamic libraries it doesn't discard them during linking phase if 
at the point of symbol resolution they don't provide any needed symbols.