Fix usage of DESTDIR in rbconfig for 1.8.6
Fix usage of DESTDIR in rbconfig for 1.8.6

  Appeared in 1.8.6-p383, reported to Ruby-core:
  Bug #1941 [ruby-core:24934]
  http://redmine.ruby-lang.org/issues/show/1941

leonhooijer/rubyinstaller,jahanzaibbahadur/rubyinstaller,vemmaverve/rubyinstaller,zorosteven/rubyinstaller,modulexcite/rubyinstaller,vemmaverve/rubyinstaller,haonaturel/rubyinstaller,dreamauya/rubyinstaller,leonhooijer/rubyinstaller,auyaauya/rubyinstaller,Rugbyman21/rubyinstaller,haonaturel/rubyinstaller,shirosaki/rubyinstaller,haonaturel/rubyinstaller,suvarnaraju/rubyinstaller,suvarnaraju/rubyinstaller,zorosteven/rubyinstaller,dreamauya/rubyinstaller,jaym/rubyinstaller,modulexcite/rubyinstaller,modulexcite/rubyinstaller,jaym/rubyinstaller,shirosaki/rubyinstaller,jahanzaibbahadur/rubyinstaller,shirosaki/rubyinstaller,Rugbyman21/rubyinstaller,zorosteven/rubyinstaller,vemmaverve/rubyinstaller,oneclick/rubyinstaller,jaym/rubyinstaller,auyaauya/rubyinstaller,leonhooijer/rubyinstaller,Rugbyman21/rubyinstaller,auyaauya/rubyinstaller,oneclick/rubyinstaller,suvarnaraju/rubyinstaller,jahanzaibbahadur/rubyinstaller,dreamauya/rubyinstaller,oneclick/rubyinstaller