configure.in: non-portable shell behavior

* configure.in (unexpand_shvar): get rid of non-portable shell
  behavior on OpenBSD, so no extra quotes.  [Bug #7959]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39506 b2dd03c8-39d4-4d8f-98ff-823fe69b080e