Add Cilk runtime version 2856
Add CMake support for the cilkplus runtime
- Build a shared library out of the source
- Install the cilk plus header files into clang
- Support for Linux and Darwin on x86_64
- Link to dl, pthreads to be consistent with ICC-distributed libraries
Install cilk headers and libcilkrts into regular include, lib directories

- No longer in internal lib/clang/3.3/* locations
- Update to validation suite in separate patch

Reviewed by Andy Zhang
Add LICENSE.txt for Cilk Plus Runtime
Support libcilkrts and cilk headers with autoconf