Every 'extern' declaration must have a corresponding definition somewhere - this is what "resolves" the external reference.

You should take the _at_ off your 'extern' declarations in the header file, and put it into the definitions elsewhere in your project.

blah.h
EXTERN xdata volatile BYTE PORTBCFG; 
EXTERN xdata volatile BYTE PORTCCFG; 
EXTERN xdata volatile BYTE OUTB; 
EXTERN xdata volatile BYTE OUTC; 
EXTERN xdata volatile BYTE OEB; 
EXTERN xdata volatile BYTE OEC; 

blah.c
#include "blah.h" // this allows the compiler to spot any inconsistencies!

xdata volatile BYTE PORTBCFG _AT_ 0x7F94;
xdata volatile BYTE PORTCCFG _AT_ 0x7F95;
xdata volatile BYTE OUTB _AT_ 0x7F97;
xdata volatile BYTE OUTC _AT_ 0x7F98;
xdata volatile BYTE OEB _AT_ 0x7F9D;
xdata volatile BYTE OEC _AT_ 0x7F9E;