how to use libpng?

hi all,
I want to know how to use libpng.

when I tried to use the example.c , I got some linker error( I am using gc++).
ex:
[Linker error] undefined reference to `png_sig_cmp'
[Linker error] undefined reference to `png_create_read_struct'
[Linker error] undefined reference to `png_create_info_struct'
......

I have put zlib and libpng in my work folder
work folder -> zilb -> all files unzipped from zlib
work folder -> libpng -> all files unzipped from libpng1.2.8

in my code,

include "zlib\zlib.h"
include "libpng\libpng.h"

did I do something wrong??