Make sure that userspace is always referencing "system" headers in a way
that would build on target :). This means removing the explicit
include_directories of Libraries/LibC in favor of having it export its
headers as SYSTEM. Also remove a redundant include_directories of
Libraries in the 'serenity build' part of the build script. It's already
set at the top.
This causes issues for the Kernel, and for crt0.o. These special cases
are handled individually.
This stopped working quite some time ago due to Clang losing track of
typestates for some reason and everything becoming "unknown".
Since we're primarily using GCC anyway, it doesn't seem worth it to try
and maintain this non-working experiment for a secondary compiler.
Also it doesn't look like the Clang team is actively maintaining this
flag anyway. So good-bye, -Wconsumed. :/
