- 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.
The benefits are primarily being able to use FetchContent, which allows
for a more flexible dependency handling. Other various quality-of-life
features such as `-B` and `-S` flags are also included.
This also removes broken `--version` generation as it does not work for
version 3.10 and 3.11 due to the `JOIN` generator expression.
Reference: https://github.com/neovim/neovim/issues/24004
