configure.in: quite conftest

* configure.in (rb_cv_broken_backtrace): suppress extra output from
  conftest.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39367 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
un.rb: suppress warning

* lib/un.rb (wait_writable): show error messages only if -v is given.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39368 b2dd03c8-39d4-4d8f-98ff-823fe69b080e