`clingo` has some undocumented dependencies:
- `bison`, for which the default macos version is too old
- `re2c`, which isn't always available
Also, the python installation was not set up properly. Clingo by default
does an install in `~/.local`, so we need to disable that and add a few
other options to get things right.
- [x] add `bison` dependency
- [x] add `re2c` dependency
- [x] make `doxygen` dependency optional (and patch if needed)
- [x] add options to fix `python` build
- [x] make python build optional but on by default
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