Add include-what-you-use

Builds against clang 3.5. See [their homepage][1] for more information.

[1]: https://code.google.com/p/include-what-you-use/
Add symlink to handle builtin headers

IWYU complains about not being able to find standard headers which are
built into Clang. See the [the Clang documentation][1] for more.

Also fixed some path problems.

[1]: http://clang.llvm.org/docs/LibTooling.html#libtooling-builtin-includes
Fix style issues

It appears homebrew doesn't like "new"-style hash literals.
Force older OS X versions to use the right stdlib

I think this should fix (Mountain) Lion builds.
include-what-you-use: add 0.3 bottle.