OSX: fix compilation with 10.7 SDK

Recent work in the OS X parts of the code started using clang's support for
Obj-C's support for Literals and Subscripting. These particular language
features remove a lot of boilerplate code and allow to interact with
collections as consicely as one would do in scripting languages like Ruby or
Python.

Even if these are compiler features, Subscripting needs some runtime support.
This is provided with libarclite (coming with the compiler), but we need to
add the proper method definitions since the 10.7 SDK headers do not include
them. That is because 10.7 shipped before this language features.

This will cause some warnings when compiling with the 10.7 SDK because the
commit also redefines BOOL to make autoboxing/unboxing of BOOL literals to
work.

If you need to test this for whatever reason on 10.8, just pass in the correct
SDK to configure's extra cflags:

  ./configure --extra-cflags='-mmacosx-version-min=10.7 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.7.sdk'

Fixes #117