opencascade-occt: Build with rapidjson to enable glTF export in FreeCAD

This is to enable glTF export support in FreeCAD. FreeCAD depends on
the JSON support in OpenCascade to be able to export to
glTF (https://forum.freecad.org/viewtopic.php?t=63440).

And it turns out that OpenCascade doesn't default to having this
enabled by default:
https://dev.opencascade.org/content/building-linux-rapidjson

pkgs/by-name/op/opencascade-occt/package.nix

@@ -5,6 +5,7 @@
   fetchpatch,
   cmake,
   ninja,
+  rapidjson,
   tcl,
   tk,
   libGL,
@@ -48,9 +49,11 @@ stdenv.mkDerivation rec {
     libXext
     libXmu
     libXi
+    rapidjson
   ] ++ lib.optional stdenv.hostPlatform.isDarwin darwin.apple_sdk.frameworks.Cocoa;
 
   NIX_CFLAGS_COMPILE = [ "-fpermissive" ];
+  cmakeFlags = [ "-DUSE_RAPIDJSON=ON" ];
 
   meta = with lib; {
     description = "Open CASCADE Technology, libraries for 3D modeling and numerical simulation";