dl/extconf.rb: check for -fno-defer-pop option

* ext/dl/extconf.rb: check for -fno-defer-pop option, since clang
  5.1 no longer support -fno-defer-pop option.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45365 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-03-20

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45366 b2dd03c8-39d4-4d8f-98ff-823fe69b080e