Make Graphviz buildable as a cmake subproject/subdirectory
Sometimes it might be convenient to just add graphviz as a subdirectory and manually build it in your cmakelists, however, it looks like currently it's prevented by the use of
CMAKE_SOURCE_DIR in a few places (which would link to the topmost CMakeLists.txt), when it should've been
CMAKE_CURRENT_SOURCE_DIR (so relative paths would be correct). Maybe there are other issues, I haven't investigated further.