ho aggiunto -Wall nel file make file così:

CC = gcc -Wall
SRC = main.c more.c command.c
DEF = defs.h
OBJ = $(SRC:.c=.o)


questo è il mio risultato della compilazione..

paul@linux:~/Desktop/prova> make
gcc -Wall -c -o main.o main.c
main.c: In function ‘main’:
main.c:17: warning: implicit declaration of function ‘more’
main.c:22: warning: implicit declaration of function ‘exit’
main.c:22: warning: incompatible implicit declaration of built-in function ‘exit’
gcc -Wall -c -o more.o more.c
more.c: In function ‘more’:
more.c:22: warning: implicit declaration of function ‘command’
more.c:28:3: warning: no newline at end of file
gcc -Wall -c -o command.o command.c
command.c:37:3: warning: no newline at end of file
make: *** No rule to make target `gcc', needed by `More'. Stop.

però ancora non va..