Generating C code from a Matlab-Simulink: errors encountered during linking
Typical unresolved symbols appear because the code (written/generated) is referencing external methods or types that are well DECLARED but its DEFINITIONS are not provided.
Declarations are commonly in .h files included in the source code.
How you provide this definitions? 
- You need pass the LIBRARY containing these definitions during the linker pass of the build process. Libraries are .a or .lib files depending of the compiler used. 
- Other way to pass these definitions is include other .c or .cpp files in the compilation process that define these symbols (methods, types, vars).

In your case, you need check the CSS documentation to resolve if you need code your own .c/.cpp or where you can get the .a/.lib library files or may be you need check the path of the libraries.