Request: make this work with dune!

[Tom Ridge]

Hi,

This ppx doesn't seem to work with dune.

dune build @install
File "dune", line 4, characters 14-30:
4 |   (preprocess (pps ppx_getenv))
                  ^^^^^^^^^^^^^^^^
Error: No ppx driver were found. It seems that ppx_getenv is not compatible
with Dune. Examples of ppx rewriters that are compatible with Dune are ones
using ocaml-migrate-parsetree, ppxlib or ppx_driver.
File "dune", line 11, characters 14-30:
11 |   (preprocess (pps ppx_getenv))
                   ^^^^^^^^^^^^^^^^
Error: No ppx driver were found. It seems that ppx_getenv is not compatible
with Dune. Examples of ppx rewriters that are compatible with Dune are ones
using ocaml-migrate-parsetree, ppxlib or ppx_driver.
Makefile.ocaml:16: recipe for target 'build' failed

Are there any alternatives? I want to do conditional compilation based on environment variables.