[CMake] Allow building standalone without any llvm-config available
[CMake] Allow building standalone without any llvm-config available

This is the same as libcxxabi/libcxx do.

Differential Revision: https://reviews.llvm.org/D50135

git-svn-id: 62ef012455b6b8fcec9c41ca270ebf409aae1006@338819 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libunwind,llvm-mirror/libunwind,llvm-mirror/libunwind