lnx1 1> make texternal
pgf77 -g -c texternal.f
pgf77 -g -L/usr/local/PGI/lib texternal.o -o texternal

lnx1 2> texternal
 r8caller:    100.0000000000000     
 r8sub: x =    10.00000000000000      y =    20.00000000000000