zlib.h problem

Siden jeg ikke har haft så meget respons på LQ, prøver jeg lige her:
m trying to use the zlib library and it seems like it is included and found (when I compile the c file using the zlib.h, there's no error). Never the less, when I link my program I get a undefined reference, to the function I use i zlib (compress2).

    code:

//gzip_interface.c

#include <string.h>
#include <stdlib.h>
#include <linux/zlib.h>
#include "gzip_interface.h"

int compress_buf(char *dst, char *src, unsigned short int level )
{
    int size = strlen(src);
    int return_size = 0;
    dst = malloc(size);
    return_size = compress2(dst, size, src, size, level);
    return 0;
   
}

//gzip_interface.h

#ifndef GZIP_INTERFACE
#define GZIP_INTERFACE

int compress_buf(char *dst, char *src, unsigned short int level);
#endif




How can this be? I've search for zlib, and it is located in /usr/include and also in /usr/include/linux. I've tried including it like this: #include <zlib.h>, but it's the same problem. There's no problem including the library, but it doesn't seem to find the functions. I'm using Anjuta (on ProMepis 2005.b04), and I've tried adding the library manually, so it links with -lzlib option.. that didn't work either (Second Q: How do you know when it need to be included manually when linking? .. e.g. pthread needs to : -lpthread ??).
Any help will be greatly appriciated.

Regards jnusa