Make it clear we expect a path as first argument to preprocess.rb

Author: eregon

lib/cext/preprocess.rb

@@ -61,7 +61,7 @@ def self.patch(file, contents, directory)
   if __FILE__ == $0
     require 'stringio'
 
-    file_name = ARGF.filename
+    file_name = ARGV.first
     original_content = File.read(file_name)
     output = patch(file_name, original_content, Dir.pwd)