fix warning message where it would display "ld: warning: ObjC object file xxxx"

The warning message will be displayed when use Xcode 5 even if deployment_target is not iOS 7.
This will fix https://github.com/HipByte/RubyMotion/commit/c2fc9959edb7fecc1a24f345d62cafe686aa4960

% rake
     Build ./build/iPhoneSimulator-6.1-Development
   Compile ./app/app_delegate.rb
    Create ./build/iPhoneSimulator-6.1-Development/hello.app
      Link ./build/iPhoneSimulator-6.1-Development/hello.app/hello
ld: warning: ObjC object file (./build/iPhoneSimulator-6.1-Development/objs/init.o) was compiled for iOS Simulator, but linking for MacOSX