Bug#818787: doxygen: Changes default HAVE_DOT to YES without having graphviz in the Depends line.

[Santiago Vila]

Note 2: The easy fix does not necessariñy have to be the right fix.

As Bas Wijnen points out in -devel:

> If the program can be used without it, it should not be a Depends.  That's 
> what
> Recommends are for.