FindGraphviz.cmake: Introduce GRAPHVIZ_ROOT

Input variable to specify graphviz location
FindGraphviz.cmake: Detect even without graph lib

Still detect Graphviz, even if 'graph' library is missing. This library
is no longer built by default, since Graphviz v2.30.

Also enable NO_DEFAULT_PATH option for both find_path and find_library,
in case GRAPHVIZ_ROOT was specified. This is in order to avoid detecting
e.g. 'libcgraph.so' (from a custom install) in $GRAPHVIZ_ROOT/lib and 'libgraph.so' (from
another Graphviz install) in /usr/lib.