+_UNSUPPORTED_SYSTEM_MESSAGE= ${OPSYS} version you are running is not supported by the Ports Collection, no port is guaranteed to build. Please upgrade to a supported release or, as a last resort, use a ports tree snapshot dated within your system support lifetime.
+. if defined(ALLOW_UNSUPPORTED_SYSTEM)
+WARNING+= "${_UNSUPPORTED_SYSTEM_MESSAGE}"
+. else
+UNSUPPORTED_SYSTEM_ERROR= ${_UNSUPPORTED_SYSTEM_MESSAGE} You may define ALLOW_UNSUPPORTED_SYSTEM to proceed, but you have been warned.