cmake: Simplify and improve FindGraphviz.cmake
Remove a lot of cruft, and move detection of dot(1) to a separate
FindDOT.cmake. More specifically:
- Read Graphviz's version from a header file instead of invoking dot during configuration. dot should only be needed at runtime. Doing so allows us to significantly trim down FindGraphviz.cmake.
- State that Graphviz 2.30 is the minimum version we support in CMakeLists.txt, so that FindGraphviz.cmake remains more generic.
- Always try to invoke pkg-config in FindGraphviz.cmake, there is no need to avoid it on Windows, we just don't use its output if it is not present.
- Remove a bunch of hardcoded locations from find_library(), let CMake handle those automatically and just provide pkg-config's output as HINTS.
- Create a FindDOT.cmake that looks for dot, and lists it as a runtime dependency in CMake's summary.
Test Plan: Everything still worked fine.
Reviewers: kossebau, jriddell, kleag, adridg
Differential Revision: https://phabricator.kde.org/D14146