rebuilding occt with rapidjson

Hi ,
I'm trying to export to gltf and figured out i need to rebuild occt with rapidjson .
I have rapidjson installed and use it often when writing c++ code .
when i built opencascade in the first time , i installed all the other 3rd party libraries with apt (i'm working on ubuntu 20.04) and just did cmake to build occt and then created a CMakeList.txt file to use the library in my own project .
now i'm stuck on how to rebuild it with rapidjson . i'm new to cmake and some help would be more than welcomed .