cmake: Absolute paths in diagnostic messages
Currently CMake's diagnostic messages express paths inside the source and build trees relative to their top, e.g.
CMake Error at src/CMakeLists.txt:1:
...
These are not hard to understand for humans, at least if they know the rule, but tooling such as an IDE needs special logic to help users jump to the line in question. We should consider converting these to absolute paths, at least optionally.