[libunwind][CMake] Use libc++ headers when available

libunwind depends on C++ library headers. When building libunwind
as part of LLVM and libc++ is available, use its headers.

Differential Revision: https://reviews.llvm.org/D29997

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@295285 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed
tree: 8549b6c6adb2bf8eae508f72713f3a3b728f5aa0
  1. cmake/
  2. include/
  3. src/
  4. test/
  5. .arcconfig
  6. .clang-format
  7. CMakeLists.txt