Index: share/mk/bsd.sys.mk
===================================================================
--- share/mk/bsd.sys.mk
+++ share/mk/bsd.sys.mk
@@ -199,6 +199,14 @@
 .endif
 .endif	# gcc
 
+# There are currently lots of -Wunused-but-set-variable instances. Downgrade
+# the error to a warning until they have all been fixed.
+.if ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 130000
+# TODO: when all warnings have been fixed we should be able to turn this warning
+# on for GCC as well.
+CWARNFLAGS+=	-Wno-error=unused-but-set-variable
+.endif
+
 # How to handle FreeBSD custom printf format specifiers.
 .if ${COMPILER_TYPE} == "clang"
 FORMAT_EXTENSIONS=	-D__printf__=__freebsd_kprintf__