Move apple_notifications.mm somewhere easy to find for scons.
Require __APPLE__ to build apple_notification.mm in case another compiler runs into it.
File assumes the presence of many OS X specific headers and definitions.
Speculative change to support homebrew.
I don't actually have it, so hopefully it works.
Conditionally added because I don't know if all compilers even hope to compile
Objective-C++ and I don't want to incidentally break other builds.
Fix typo which put comment partially on the wrong line.