Make CMake get Graphviz version from autogen.sh by parsing it

Make CMake get Graphviz version from autogen.sh by parsing it

This will make autotools and CMake builds have a single source of
truth for the Graphviz version.

First, the major, minor and patch versions are retrieved. If the patch
version is "$GRAPHVIZ_VERSION_DATE", the patch version is set to the
committer date of the latest commit. This effectively repeals part of
the intention of 266ff5ee80b2d91231907e90ec650154f5225aa5: "Don't
encode build date stamp in it to be able to create reproducable
builds".