thanks to @whitequark, could use cppo for preprocessing files

AUTHORS.md

@@ -4,4 +4,4 @@
 - Drup (Gabriel Radanne)
 - Jacques-Pascal Deplaix
 - Nicolas Braud-Santoni
-
+- Whitequark (Peter Zotov)

_oasis

@@ -8,7 +8,7 @@ LicenseFile: LICENSE
 Plugins:     META (0.3), DevFiles (0.3)
 OCamlVersion: >= 4.00.1
 BuildTools:  ocamlbuild
-AlphaFeatures: compiled_setup_ml
+AlphaFeatures: ocamlbuild_more_args, compiled_setup_ml
 
 Synopsis:    A modular standard library focused on data structures.
 Description:

configure

@@ -1,7 +1,7 @@
 #!/bin/sh
 
 # OASIS_START
-# DO NOT EDIT (digest: 82230d61386befb40bc7377608e1f16e)
+# DO NOT EDIT (digest: 6f7b8221311e800a7093dc3b793f67ca)
 set -e
 
 FST=true
@@ -23,5 +23,5 @@ for i in "$@"; do
   esac
 done
 
-make configure CONFIGUREFLAGS="$@"
+make configure CONFIGUREFLAGS="$*"
 # OASIS_STOP

myocamlbuild.ml

@@ -0,0 +1,47 @@
+(* OASIS_START *)
+(* OASIS_STOP *)
+
+open Ocamlbuild_plugin;;
+
+dispatch
+  (MyOCamlbuildBase.dispatch_combine [
+    begin function
+    | After_rules ->
+      (* replace with Ocamlbuild_cppo.dispatch when 4.00 is not supported
+         anymore *)
+      let dep = "%(name).cppo.ml" in
+      let prod1 = "%(name: <*> and not <*.cppo>).ml" in
+      let prod2 = "%(name: <**/*> and not <**/*.cppo>).ml" in
+      let f prod env _build =
+        let dep = env dep in
+        let prod = env prod in
+        let tags = tags_of_pathname prod ++ "cppo" in
+        Cmd (S[A "cppo"; T tags; S [A "-o"; P prod]; P dep ])
+      in
+      rule "cppo1" ~dep ~prod:prod1 (f prod1) ;
+      rule "cppo2" ~dep ~prod:prod2 (f prod2) ;
+      pflag ["cppo"] "cppo_D" (fun s -> S [A "-D"; A s]) ;
+      pflag ["cppo"] "cppo_U" (fun s -> S [A "-U"; A s]) ;
+      pflag ["cppo"] "cppo_I" (fun s ->
+        if Pathname.is_directory s then S [A "-I"; P s]
+        else S [A "-I"; P (Pathname.dirname s)]
+      ) ;
+      pdep ["cppo"] "cppo_I" (fun s ->
+        if Pathname.is_directory s then [] else [s]) ;
+      flag ["cppo"; "cppo_q"] (A "-q") ;
+      flag ["cppo"; "cppo_s"] (A "-s") ;
+      flag ["cppo"; "cppo_n"] (A "-n") ;
+      pflag ["cppo"] "cppo_x" (fun s -> S [A "-x"; A s]);
+      (* end replace *)
+
+      let major, minor = Scanf.sscanf Sys.ocaml_version "%d.%d.%d"
+        (fun major minor patchlevel -> major, minor)
+      in
+      let ocaml_major = "OCAML_MAJOR " ^ string_of_int major in
+      let ocaml_minor = "OCAML_MINOR " ^ string_of_int minor in
+
+      flag ["cppo"] & S[A"-D"; A ocaml_major; A"-D"; A ocaml_minor]
+    | _ -> ()
+    end;
+    dispatch_default
+  ])