on SLC5 the default, shared library only, grahpviz rpm does not come with

libgvplugin_dot_layout.so and this is not needed either as graphviz will
load it dynamically, so don't explicitly link against it.


git-svn-id: http://root.cern.ch/svn/root/trunk@31334 27541ba8-7e3a-0410-8455-c3a389f83636