build: cmake fixes

- 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.

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