add String#absolute_path?

lib/rscons/environment.rb

@@ -99,7 +99,7 @@ module Rscons
         end
       end
       if @build_root and not found_match
-        unless source_fname.start_with?('/') or source_fname =~ %r{^\w:[\\/]}
+        unless source_fname.absolute_path?
           build_fname = "#{@build_root}/#{build_fname}"
         end
       end

lib/rscons/monkey/string.rb

@@ -17,4 +17,9 @@ class String
   def set_suffix(suffix = '')
     sub(/\.[^.]*$/, suffix)
   end
+
+  # Return whether the string represents an absolute filesystem path
+  def absolute_path?
+    self =~ %r{^(/|\w:[\\/])}
+  end
 end