* configure.in (AC_ARG_WITH): use withval directly.
  fix failure on FreeBSD.
  http://fb32.rubyci.org/~chkbuild/ruby-trunk/log/20131217T070301Z.diff.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44295 b2dd03c8-39d4-4d8f-98ff-823fe69b080e