Bug 13979: [3.18.x] updates to allow installation on jessie

Bug 13979: [3.18.x] updates to allow installation on jessie

This patch makes the build script keep the shipped YUI JavaScript library
instead of explicitly deleting it and using the one the operating system
provides.

Development is done against the YUI library we ship, so this makes sense
even if Debian still shipped it.

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>