How do I build the doc with OpenCV 3.1?

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I am trying to build the documents, after installation on Ubuntu 16.04, following instructions at: http://docs.opencv.org/3.1.0/d7/d9f/t...

At step 7 I got:

> pwd
/home/fanta/opt/opencv-3.1.0/build/doc
> make -j7 html_docs
make: *** No rule to make target 'html_docs'. Stop.

I have tried also with the doxygen tutorial, and I got the same kind of error (after running cmake):

> pwd
/home/fanta/opt/opencv-3.1.0/build
> make doxygen
make: *** No rule to make target 'doxygen'. Stop.
> cd doc
> make doxygen
make: *** No rule to make target 'doxygen'. Stop.

How can I build the OpenCV documentation?

Thanks!