"Always use `find_package` with `REQUIRED`."
- We make an exception for LuaJit (not REQUIRED): the `nvim-test` target
is included only if we can find LuaJit.
This is partially a cargo-cult (reference below), but it uncovered at
least one problem: `find_package(LibIntl REQUIRED)` fails on my vanilla
ubuntu 16.04 system.
ref: https://schneide.blog/2017/11/06/4-tips-for-better-cmake/
> optional dependencies is nice, but skipping on REQUIRED is not the way
> you want to do it. In the worst case, some of your features will just
> not work if those packages are not found, with no explanation
> whatsoever. Instead, use explicit feature-toggles (e.g. using option())
> that either skip the find_package call or use it with REQUIRED, so the
> user will know that another lib is needed for this feature.