WIP: Add CMake flag for OpenMP support

[josephsnyder]

If OpenMP support is selected, download the OMP source code which matches the LLVM/Clang version. Add a step to the project to extract the source to the binary directory and add the two external source directories to the compilation of LLVM.

This removes the control of the source code from the ExternalProject_Add for clang since in order to build the OpenMP code, I seem to need the source of Clang. But, the clang external project depends on LLVM.