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>
2 files changed