The SDL port failed to build because the CMake toolchain filed pointed
to the old root. Now the toolchain file assumes that the Root is in
Build/Root.
Additionally, the AK/ and Kernel/ headers need to be installed in the
root too.
This makes out-of-tree linking possible. And at the same time, add a
CMakeToolchain.txt file that can be used to build arbitrary cmake-using
applications on Serenity by pointing to the CMAKE_TOOLCHAIN_FILE when
running cmake:
-DCMAKE_TOOLCHAIN_FILE=~/code/serenity/Toolchain/CMakeToolchain.txt
