The merge request !2848 added code to automatically detect the module
prefix on macOS, with a test for the Mac #define TARGET_OS_OSX. However,
older versions of the SDK (at least 10.11) don't provide this #define,
leading to build failure. If the #define is missing, fall back to
checking TARGET_OS_MAC. On newer SDKs this symbol is also true for
watchOS, etc., but in those situations TARGET_OS_OSX is available.