The ACUCOBOL-GT runtime library (used to relink the runtime system) comes with its own main routine. The library contains this routine in a separate object module. To create a runtime system that uses your own main routine, you simply add a routine called "main" to one of your C source files and re-create the runtime. Because the linker uses your C object files before searching the library, your main routine is used and the one in the library is not linked.
To relink the runtime system, follow the instructions in the topic Relinking the Runtime System. The technique for relinking the runtime depends on the machine type.