-Wno-unused-value [ci skip]

Without this option clang outputs tons of warnings, which
are annoying. See https://travis-ci.org/ruby/ruby/jobs/456553420#L1817


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65795 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.ac

@@ -510,6 +510,7 @@ AS_IF([test "$GCC:${warnflags+set}:no" = yes::no], [
 		 -Wno-self-assign \
 		 -Wno-tautological-compare \
 		 -Wno-unused-parameter \
+		 -Wno-unused-value \
 		 -Wsuggest-attribute=format \
 		 -Wsuggest-attribute=noreturn \
 		 -Wunused-variable \