build.log

user@ubuntu:~/test$ ocamlopt -c test.c
user@ubuntu:~/test$ ocamlopt -c main.ml 
user@ubuntu:~/test$ ocamlopt -cclib -lz main.cmx test.o -o binary
test.o: In function `main':
/home/user/test/test.c:5: undefined reference to `deflateInit_'
collect2: error: ld returned 1 exit status
File "caml_startup", line 1:
Error: Error during linking

main.ml

let _ = Printf.printf "hello\n"

test.c

#include "zlib.h"

void cstub(void){
	deflateInit(NULL, 0);
}