[PATCH] Create cmake install targets for lit, FileCheck, etc. when a defaulted-to-off flag is enabled.

[Daniel Dunbar]

I think that the installation process is simpler (`easy_install lit` or
`pip install lit` on many a modern system, vs installing or checkout of
LLVM), and more appropriate for people who want lit as a standalone project.

 - Daniel

http://reviews.llvm.org/D6469