On Debian/lenny, which is a "supported" platform, ffi.h is installed
in /usr/include/i486-linux-gnu/. This causes build error when using
gcc whose target is not i486-linux-gnu.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@28332 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* test/fiddle/*: testing fiddle extension
* ext/dl/lib/dl.rb: Requiring fiddle if it is available
* ext/dl/lib/dl/callback.rb: using Fiddle if it is available
* ext/dl/lib/dl/func.rb: ditto
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@27640 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
