That won't work to give you a kernel module though...you'd be missing some paths and flags. Looking at the very bottom of BbSerial.c is instructive...

Code:

---

/*

  Local variables:

  compile-command: "gcc -D__KERNEL__ -I../../include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer \

  -fno-strict-aliasing -pipe -fno-strength-reduce -march=i586 -DMODULE -DMODVERSIONS \ 

  -include ../../include/linux/modversions.h   -DEXPORT_SYMTAB -c serial.c"

  End:

*/

---

This matches the code in the bottom of serial.c in a 2.2 series linux kernel (/usr/src/linux/drivers/char/serial.c) and I suspect a 2.4 series version too.
This suggests that BbSerial.c is intended to be compiled in the same fashion as serial.c, ie place the files into the drivers/char subdirectory of your linux source tree, and add BbSerial.o as a target to that directory's Makefile, using serial.c as a template.