Index: Mk/bsd.port.mk
===================================================================
--- Mk/bsd.port.mk
+++ Mk/bsd.port.mk
@@ -1144,6 +1144,15 @@
 .endif
 _EXPORTED_VARS+=	OSVERSION
 
+.if ${OPSYS} == FreeBSD && (${OSVERSION} < 1003000 || (${OSVERSION} >= 1100000 && ${OSVERSION} < 1100122))
+_UNSUPPORTED_MESSAGE=	${OPSYS} version you are running is not supported by the Ports Collection, no port is guaranteed to build. Please upgrade to supported release or, as a last resort, use a ports tree snapshot dated within your system support lifetime.
+. if defined(ALLOW_UNSUPPORTED_SYSTEM)
+WARNING+=	"${_UNSUPPORTED_MESSAGE}"
+. else
+IGNORE=		${_UNSUPPORTED_MESSAGE} You may define ALLOW_UNSUPPORTED_SYSTEM to proceed, but you have been warned
+. endif
+.endif
+
 # Convert OSVERSION to major release number
 _OSVERSION_MAJOR=	${OSVERSION:C/([0-9]?[0-9])([0-9][0-9])[0-9]{3}/\1/}
 # Sanity checks for chroot/jail building.