Decouple runtime/CMakeLists.txt from root CMakeLists.txt

[kinke]

There are a bunch of dependencies which currently prevent the runtimes from easily being built in a stand-alone fashion. Being able to do that would greatly help in

• building the runtime libs for a cross-compilation target, and
• building them with LTO support.

We could ship with a script downloading the full LDC source (~6 MB compressed), requiring CMake, make and a C compiler and building the runtime libs in a stand-alone fashion for a user, without requiring LLVM.