graphviz: allow to disable per target graphs (#14746)

In CMakeGraphVizOptions.cmake, allow the options GRAPHVIZ_GENERATE_PER_TARGET
and GRAPHVIZ_GENERATE_DEPENDERS to enable the generation of per target graphs
and subgraphs respectively. Both options are TRUE per default to maintain
current behavior.
Merge topic 'graphviz-one-file' into next

18bef4cd graphviz: allow to disable per target graphs (#14746)