remove String#absolute_path? monkey patch

lib/rscons/environment.rb

@@ -111,7 +111,7 @@ module Rscons
         end
       end
       if @build_root and not found_match
-        unless source_fname.absolute_path? or build_fname.start_with?("#{@build_root}/")
+        unless Rscons.absolute_path?(source_fname) or build_fname.start_with?("#{@build_root}/")
           build_fname = "#{@build_root}/#{build_fname}"
         end
       end