ZIP Module - Use the system minizip when available

- Ship the vlc module header zip.h
 (rename isn't needed once the file in ship)
- Detect minizip presence with pkgconfig
- if .pc not available, try to assume presence from unzip.h
- Move the zlib.h detection in one place (after pkgconfig macro)
 minizip in bundled within zlib in some distro
- Adds -lz in module that needs it in one place
- Prevent the compilation of libs/unzip if minizip is available
 (it shound't prevent this directory to be bundled with the sources)

Signed-off-by: Rémi Denis-Courmont <rdenis@simphalempin.com>