llvm: More improvements, especially for Snow Leopard

* Set MACOSX_DEPLOYMENT_TARGET properly, which is needed for building on
  Snow Leopard.
* Build libc++abi on Snow Leopard.
  - Snow Leopard is not shipped with libc++abi, which is needed to build
    libc++.
  - Set rpath to save user from setting DYLD_LIBRARY_PATH.
  - Ship cxxabi.h and libc++abi.dylib to proper locations.
* LLVM 3.3 specific changes:
  - No 3.3 release of libc++abi is available. Using latest 3.2 branch of
    libc++abi for llvm 3.3 instead.
  - For llvm 3.3, unwind.h shipped with clang doesn't provide sufficient
    definitions to build libc++abi. Backport patch from trunk to fix
    this.
* Strengthen options dependencies.
  - "--with-libcxx" requires "--with-clang".
  - "--with-libcxx" requires "--rtti".
* Refine environmental variable settings in caveats.
Add formula for Plowshare 3

Closes #224.

Signed-off-by: Xiyue Deng <manphiz@gmail.com>