[CMake] Get libunwind building under LLVM/runtimes

The new LLVM runtimes directory requires the same conventions to be
followed across the runtime projects. These changes make libunwind
build under the runtimes subdirectory.

This patch contains the following changes:

* Rename LLVM_CONFIG to LLVM_CONFIG_PATH
* Check if compiler supports C++11 (required by libunwind)

Differential Revision: https://reviews.llvm.org/D26360

llvm-svn: 286308