DIP 43: D/Objective-C

Here it finally is: support for `extern(Objective-C)`. The matching pull request for druntime is here: https://github.com/D-Programming-Language/druntime/pull/957

http://wiki.dlang.org/DIP43
You've changed the return value from bool to int, yet the non-``DMD_OBJC`` portions still return ``true``.