The problem lies in setting DV3RAP=TRUE.
When TRUE, the command to compile exrap.c is:

gcc -c -DV3RAP exrap.c

when the command to compile exrap.c is suppose to be:

gcc -ansi -pedantic -Wall -ffloat-store -fsigned-char -O -DUNIX -DVARKON -I/usr/X11R6/include -c -o exrap.o exrap.c

with what ever DV3RAP is set to; otherwise, UNIX does not get set and the compiler cannot find GL.h