yoba@mamka_tvoya:/home/yoba/c/896120 % ls *
Makefile        foo.c           main.c

include:
foo.h
yoba@mamka_tvoya:/home/yoba/c/896120 % cat Makefile
compiler = gcc
inc = ./include

main: main.o foo.o
        $(compiler) main.o foo.o -o ./main

main.o: main.c
        $(compiler) -I$(inc) -c main.c -o main.o

foo.o: foo.c
        $(compiler) -I$(inc) -c foo.c -o foo.o
yoba@mamka_tvoya:/home/yoba/c/896120 % make
gcc -I./include -c main.c -o main.o
gcc -I./include -c foo.c -o foo.o
gcc main.o foo.o -o ./main
yoba@mamka_tvoya:/home/yoba/c/896120 % ./main
main: enter
foo_ext
main: after foo
main: ext var: -42