Ignore linker warnings when compiling native extensions.

Index: lib/mkmf.rb
--- lib/mkmf.rb.orig
+++ lib/mkmf.rb
@@ -415,7 +415,7 @@ MESSAGE
         result = nil
         Logging.postpone do |log|
           output = IO.popen(env, command, &:read)
-          result = ($?.success? and File.zero?(log.path))
+          result = $?.success?
           output
         end
         result