darwin: Fix Makefile build on macOS 10.11 and older TARGET_OS_OSX is not defined when building with the clang 8 provided by Xcode 8.2.1 on 10.11.5 (unless the Xcode project is used). If it is not defined we'll just assume we have a "OSX" target. Closes #1097 Signed-off-by: Tormod Volden <debian.tormod@gmail.com>