This ports over the `LADYBIRD_USE_LLD` option from the standalone
Ladybird build and generalizes it to work for mold as well: the
`LAGOM_USE_LINKER` variable can be set to the desired name of the
linker. If it's empty, we default to trying LLD and Mold on ELF
platforms (in this order).
This means that Ladybird can be built with either Meta/Lagom or Ladybird
as the top-level source directory. This setup is a bit awkward, but will
preserve the packaging story for Ladybird until we come up with a more
permanent solution.
Tell CMake to not create a new policy scope for the
(lagom|serenity|common)_options.cmake helpers, and lets us set common
policies for both projects in common_options.cmake that actually apply
to the rest of the project, instead of just common_options.cmake itself.
This common strategy of having a serenity_option() macro defined in
either the Lagom or top level CMakeLists.txt allows us to do two things:
First, we can more clearly see which options are Serenity-specific,
Lagom-specific, or common between the target and host builds.
Second, it enables the upcoming SuperBuild changes to set() the options
in the SuperBuild's CMake cache and forward each target's options to the
corresponding ExternalProject.
