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.
