Use OCaml syntax to generate dune file

Reference aantron/bisect_ppx#294 (comment)

.github/workflows/test.yml

@@ -16,9 +16,7 @@ jobs:
           ocaml-version: ${{ matrix.ocaml-version }}
       - name: Install dependencies
         run: |
-          opam pin add routes.dev -n .
-          opam depext -yt routes
-          opam install -t . --deps-only
+          opam install dune alcotest
       - name: Run tests
         run: |
           opam exec -- dune runtest

src/dune

@@ -1,5 +1,12 @@
-(library
- (name routes)
- (public_name routes)
- (preprocess
-  (pps bisect_ppx --conditional)))
+(* -*- tuareg -*- *)
+
+let preprocess =
+  match Sys.getenv "BISECT_ENABLE" with
+  | exception _ -> ""
+  | "yes" -> "(preprocess (pps bisect_ppx))"
+  | _ -> ""
+
+let () = Jbuild_plugin.V1.send @@ {|
+ (library
+   (name routes)
+   (public_name routes)|} ^ preprocess ^ {|)|}