yoba@mamka_tvoya:/home/yoba/c/896120 % cat Makefile
CPPFLAGS=-I./include

main: main.o foo.o
yoba@mamka_tvoya:/home/yoba/c/896120 % make
cc -O2 -pipe  -c main.c
main.c:4:17: error: foo.h: No such file or directory
main.c: In function 'main':
main.c:11: error: 'bar_ext' undeclared (first use in this function)
main.c:11: error: (Each undeclared identifier is reported only once
main.c:11: error: for each function it appears in.)
*** [main.o] Error code 1

Stop in /usr/home/yoba/c/896120.
yoba@mamka_tvoya:/home/yoba/c/896120 % sed -I '' s/CPP/C/ Makefile
yoba@mamka_tvoya:/home/yoba/c/896120 % make
cc -I./include -c main.c
cc -I./include -c foo.c
yoba@mamka_tvoya:/home/yoba/c/896120 % uname -a
FreeBSD mamka_tvoya 9.3-RELEASE-p43 FreeBSD 9.3-RELEASE-p43 #0: Sat May 28 00:15:23 UTC 2016     root@amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  i386