Fix ksh/bash-ism.

* configure.in (unexpand_shvar): Use the numeric comparison
  operator instead of '==' which is a ksh extention. [Bug #7941]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39481 b2dd03c8-39d4-4d8f-98ff-823fe69b080e