[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: https://llvm.org/svn/llvm-project/libunwind/trunk@338819 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed