Expect the C headers in a different location

This should be compatible with a non Nix install of onnxruntime, and easily fixable in nix.

cbits/ort.h

@@ -1 +1 @@
-#include <onnxruntime/core/session/onnxruntime_c_api.h>
+#include <onnxruntime_c_api.h>

default.nix

@@ -14,14 +14,28 @@ let filterSource = with lib; builtins.filterSource (path: type:
       (baseNameOf path == "build" -> type != "directory") &&
       (baseNameOf path == "nix" -> type != "directory")
     );
-in
-naersk.buildPackage {
+
+    # Version of onnxruntime.dev with include directory set up the same way as
+    # a released version from microsoft/onnxruntime GitHub release.
+    onnxruntime-headers = stdenv.mkDerivation {
+      name = "onnxruntime-headers";
+      src = onnxruntime.dev;
+      installPhase = ''
+        mkdir -p $out/include
+
+        cp include/onnxruntime/core/session/onnxruntime_c_api.h $out/include/onnxruntime_c_api.h
+      '';
+    };
+
+in naersk.buildPackage {
   root = filterSource ./.;
 
   LIBCLANG_PATH = "${llvmPackages.libclang}/lib";
-  buildInputs = [ onnxruntime.dev ];
+  buildInputs = [ onnxruntime onnxruntime-headers ];
   nativeBuildInputs = [ pkg-config clang ];
 
+  # Note: This creates a doc attribute (nested derivation)
   doDoc = true;
+
   doCheck = true;
 }