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

[Dan Albert]

I think you need to not specify -DCMAKE_C(XX)?_COMPILER on the second run.
Cmake doesn't let you change compiler after the first run. Even though
you're configuring it as the same compiler, it might just reject those
options on a rerun.

http://reviews.llvm.org/D5016