Install a copy of the libc++ headers with clang.  <rdar://problem/10096516>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140876 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/runtime/Makefile b/runtime/Makefile
index 784eb66..4b0625d 100644
--- a/runtime/Makefile
+++ b/runtime/Makefile
@@ -12,7 +12,7 @@
 
 ifndef NO_RUNTIME_LIBS
 
-PARALLEL_DIRS  := compiler-rt
+PARALLEL_DIRS  := compiler-rt libcxx
 
 endif