eng_cppheader = function(options)
{
  opts = options$engine.opts
  code = paste(options$code, collapse = "n")
  if (is.null(opts$path)) opts$path = tempdir()
  cat(code, file = file.path(opts$path, opts$filename))
  Sys.setenv(PKG_CPPFLAGS = paste0("-I", opts$path))
  options$engine = 'cpp'
  engine_output(options, code, "")
}
knitr::knit_engines$set(cppheader=eng_cppheader)