Index: head/Mk/bsd.default-versions.mk
===================================================================
--- head/Mk/bsd.default-versions.mk
+++ head/Mk/bsd.default-versions.mk
@@ -15,6 +15,15 @@
 
 LOCALBASE?=	/usr/local
 
+.for lang in APACHE BDB FIREBIRD FPC GCC GHOSTSCRIPT LINUX LUA MYSQL PERL6 \
+	PGSQL PHP PYTHON PYTHON2 PYTHON3 RUBY SSL TCLTK
+.if defined(${lang}_DEFAULT)
+WARNING+=	"The variable ${lang}_DEFAULT is set and it should only be defined through DEFAULT_VERSIONS+=${lang:tl}=${${lang}_DEFAULT} in /etc/make.conf"
+WARNING+=	"This behaviour has never been supported and will be removed on 2017-01-31"
+.endif
+#.undef ${lang}_DEFAULT
+.endfor
+
 .for lang in ${DEFAULT_VERSIONS}
 _l=		${lang:C/=.*//g}
 ${_l:tu}_DEFAULT=	${lang:C/.*=//g}