Generate graphviz files at generate time, not configure time.

The feature needs access to all link libraries. In the future that
will only be possible to calculate at generate-time.

Even when the files were generated at configure time, they were
generated after user code in CMakeLists files were generated. No
policy is needed to handle manipulation of the files from CMake
code, because that was never possible.
Merge topic 'graphviz-at-generate-time' into next

6cf5564 Generate graphviz files at generate time, not configure time.