Added option to build Clang with libc++

Clang built without libc++ refers /usr/inlucde/c++/4.2.1 as include-path.
It means that a standard library is for GCC 4.2.1 and Clang can't
use many c++11 features in STL.

This commit enables clang to use latest libc++. It builds latest libc++
and applies a patch to add libc++ header path to include-paths because
Clang's include-path is hard coded.
The reason why I decide to build libc++ in spite of XCode having libc++
is that the latest Clang occurs an error with libc++ in XCode. This
error occurs because the place of `[[noreturn]]` is wrong in XCode's
libc++.

At first, libc++ in Homebrew was pull-requested by @samueljohn and the
discussion moved to homebrew-dupes, and concluded that libc++ should be
included in LLVM formula. So, I write this commit.

Thanks.

ref:
https://github.com/mxcl/homebrew/issues/13408
https://github.com/Homebrew/homebrew-dupes/pull/52