Kill enable_graphviz cmake variable

Kill enable_graphviz cmake variable
It was used to get settings from user (whether this should be
forcefully disabled or autodetected), and to get the result of
autodetection.

Now, graphviz stuff is searched all the time, and if a runable version
is found (variable HAVE_GRAPHVIZ set to 1 or 0), the dot loader of
simdag is integrated in the lib.

This is closer to the spirit of configuration stuff, IMHO.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@8696 48e7efb5-ca39-0410-a469-dd3cf9ba447f