Added option to CMake to not auto-build llvm.
Added option to CMake to not auto-build llvm.

This greatly increases build speed on build systems that have
slow dependency resolution (make, msvc), but requires you to
manually build llvm_target any time it changes.

Change-Id: I2bd3f5cd3875e6ef6477656020ba346328327b37
Reviewed-on: https://wn-project.dnsalias.net/gerrit/93
Reviewed-by: Devon Powell <bef84a74f4a3947934cc58306a6aae518eaf1936@gmail.com>
Try: Andrew Woloszyn <2926f0436f8103ab8257713335cf26447d8bf68a@google.com>
Tested-by: WNBot <efa0942020584fb66fb8bea377b59e48fa0f85ef@gmail.com>

WNProject/WNFramework,WNProject/WNFramework,WNProject/WNFramework,WNProject/WNFramework