- add EXTERNALPROJECT_OPTIONS variable to main build
- use `REQUIRED` keyword for IWYU.
- remove check_c_compiler_flag checks when `ENABLE_COMPILER_SUGGESTIONS`
is `ON`. If we explicitly enable it then we probably want it to give
an error if it doesn't exist, rather than silently skip it.
- Move dependency interface libraries to their find module and use them
as a pseudo-imported target.
- Remove BUSTED_OUTPUT_TYPE. It's not used and we can reintroduce it
again if something similar is needed.
- Use LINK_OPTIONS intead of LINK_FLAGS when generating the `--version`
output.
- Remove unused code
- Use consistent casing. Variable names such as LibLuV_LIBRARIES is
needlessly jarring, even if the name might be technically correct.
- Use title casing for packages. find_package(unibilium) requires the
find_module to be named "Findunibilium.cmake", which makes it harder
to spot when scanning the files. Instead, use "Unibilium".
