The purpose is to allow pnacl-sz to be built within an LLVM checkout (version 3.9 or later), and not require the complex PNaCl build environment.

Within an LLVM checkout, one would do something like the following:
  cd projects
  git clone https://chromium.googlesource.com/native_client/pnacl-subzero
  # change to LLVM build directory
  cmake -G "Ninja" <path_to_llvm_source_dir>
  ninja

A follow-on CL will add in the PNaCl bitcode reader source files that are needed for compiling and linking.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1961743002 .