Remove strict dependency on llvm-config.

Depending on llvm-config allows for a few nice things (auto-detecting
LLVM source tree, version numbers, etc), but it's makes bootstrapping a
pain. Keep the llvm-config features around, but also fall back to being
able to configure based on -DLLVM_PATH=path/to/llvm.

Reviewers: jroelofs, loladiro

Reviewed By: loladiro

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5016