configure.in: no Werror

* configure.in (warnflags): disable -Werror by default unless
  development.  [ruby-core:52131] [Bug #7830]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39221 b2dd03c8-39d4-4d8f-98ff-823fe69b080e