libvncserver: fix configure arg

use the correct configure arg
from configure.ac:
AC_ARG_WITH(tightvnc-filetransfer,

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>
Merge branch 'master' of https://github.com/nakarotori/packages
libpng: update to libpng 1.2.51

upstream changelog:
http://www.libpng.org/pub/png/src/libpng-1.2.51-README.txt

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>