Add a workaround for clang bug.
diff --git a/configure.in b/configure.in
index e8f931c..e4e255f 100644
--- a/configure.in
+++ b/configure.in
@@ -48,6 +48,9 @@
   ;;
 freebsd*)
   CFLAGS="$CFLAGS -pedantic -Wall -Werror -pthread"
+  if "x$CC" = "xclang"; then
+    CFLAGS="$CFLAGS -Qunused-arguments"
+  fi
   LIBCFLAGS="$LIBCFLAGS -U__FreeBSD__ -D__Userspace_os_FreeBSD"
   ;;
 solaris*)