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

[Dan Albert]

Keno, PTAL. I've fixed the following things in this revision:

 * Allow the user to specify a specific LLVM tree even if llvm-config is on their system.
 * Had accidentally removed LLVM_BINARY_DIR for llvm-config path, preventing it from finding the cmake files.
 * Fixed behavior for re-running cmake in already configured directory.

I've verified that either path works (even under a re-run of cmake) and can correctly find the cmake files, build and run LIT.

If everything works for you, please approve and I'll commit it.

http://reviews.llvm.org/D5016