The basic idea is to replace

  gcc ... file.c ...

with

  gcc -E ... file.c ... -o preproc.c
  dietc preproc.c > diet.c
  pass1 diet.c > diet.c
  pass2 diet.c > diet.c
  ...
  gcc ... diet.c ...

To assist with this, we provide `scripts/dietcc`, which provides a drop-in
replacement for `gcc` that can run your patches. In your build scripts, simply
replace `gcc` with the path of `dietcc`.

You can insert your own compilation pass into dietcc; read its Python source
for examples.