On macOS, CMake incorrectly tries to add and/or remove rpaths from files
that it has already processed when it performs installation. Setting the
rpaths during the build process ensures that they are only set once, and
as a bonus, makes installation slightly more performant.
Fixes#10055.
We use the environment variable SERENITY_SOURCE_DIR to resolve and check
icon links. This is a bit inconvenient as SERENITY_SOURCE_DIR needs to
be set correctly before invoking the markdown checker, but as we use it
through the check-markdown script anyways, I think it's not a problem.
