libvncserver: add config option for tightpng encoding support

TightPNG encoding speeds up HTML5 based VNC clients like noVNC.
libvncserver enables this encoding if both libpng and jpeg libraries
are available.

[Thomas: rewrap Config.in help text.]

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>