ppx_cstruct seems broken on 4.04

[gasche]

Two independent ppx_struct users, arp and mirage-profile, are broken on 4.04+beta on what seems to be a ppx_cstruct issue:

http://opam.ocamlpro.com/builder/html/mirage-profile/mirage-profile.0.7.0/d5a819fca03c2e5d0f175590b6022d12

+ /home/lefessan/builder/4.04.0+beta1/.opam/4.04.0+beta1/bin/ocamlfind ocamldep -package cstruct -package cstruct.ppx -package lwt -package ocplib-endian.bigstring -modules lib/trace.mli > lib/trace.mli.depends
File "lib/trace.mli", line 1:
Error: External preprocessor does not produce a valid file
Command line: ppx_cstruct '/tmp/camlppx916bcf' '/tmp/camlppx918db5'

http://opam.ocamlpro.com/builder/html/arp/arp.0.1.1/f4ae306471f10b5f44ffa3090684c677

+ ocamlfind ocamldep -package cstruct -package cstruct.ppx -package result -modules src/arp_wire.ml > src/arp_wire.ml.depends
File "src/arp_wire.ml", line 1:
Error: External preprocessor does not produce a valid file
Command line: ppx_cstruct '/tmp/camlppxeb84fe' '/tmp/camlppx986d5f'