package/pngquant: new package

package/pngquant: new package

The host package will be needed by tvheadend.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas:
 - group the host commands together and the target commands together
 - use --prefix=$(HOST_DIR)/usr for host configuration, and do not
   override PREFIX= at host install time
 - use DESTDIR=$(TARGET_DIR) instead of overriding PREFIX= at target
   install time.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>