[CMake][libcxx] Check that we have libcxxabi before using it
[CMake][libcxx] Check that we have libcxxabi before using it

When doing standalone build, check that we actually have libcxxabi
before attempting to use it.

Differential Revision: https://reviews.llvm.org/D28580

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@291723 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx