configure.in: suppress warnings

* configure.in (warnflags): suppress warnings caused by macro
  expansion with clang (and maybe -save-temps option).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53594 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -818,6 +818,7 @@ if test "$GCC:${warnflags+set}:no" = yes::no; then
 		 -Wno-tautological-compare \
 		 -Wno-parentheses-equality \
 		 -Wno-constant-logical-operand \
+		 -Wno-self-assign \
 		 -Wunused-variable \
 		 -Werror=pointer-arith \
 		 -Werror=write-strings \