Did you #include optix_stubs.h? That’s what the course samples do.
If includes are setup right, then that symbol should be defined in your program’s compiled object file, it shouldn’t need to be linked from elsewhere.
BTW does the optix7course CMake setup work for you? You can always explore what CMake is doing in gcc terms by building with VERBOSE=1 on your ‘make’ command. That will let you see all the command lines including lib & .o paths in a working case, so you can use nm to find out where symbols are coming from.