[PATCH] [libcxxabi] Remove strict dependency on llvm-config.

[Keno Fischer]

One issue is that, if there is a llvm-config executable in the path, it tried to use that, which I want to prevent (this happened to me while trying to rebuild with different options - libcxxabi wasn't installed anymore, but llvm-config depending on it still was).

http://reviews.llvm.org/D5016