Install builder: remove target file before copying to avoid errors due to overwriting read-only files - close #123

lib/rscons/builders/install.rb

@@ -47,6 +47,7 @@ module Rscons
               printed_message = true
             end
             cache.mkdir_p(File.dirname(dest))
+            FileUtils.rm_f(dest)
             FileUtils.cp(src, dest, :preserve => true)
           end
           cache.register_build(dest, :Copy, [src], env)