codegen doesn't support fit function, is there a work around way to implement same functionality?

I use codegen to generate C code from Matlab code. In Matlab code, I use fit function for a simple linear fit z=a+b*x+c*y. Also I need to provide fit options such as Lower, Upper, StartPoint. But codegen doesn't support fit function. Does anyone has a good work around way to implement this functionality anyhow?

Thanks in advance.