curses/extconf.rb: simplify

* ext/curses/extconf.rb (curses_version): try once for each tests, a
  function or a variable.  fallback to variable for old SVR4.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40538 b2dd03c8-39d4-4d8f-98ff-823fe69b080e