[Development] Retiring libtiff too

[Shawn Rutledge]

> On 3 May 2016, at 08:39, Lars Knoll <Lars.Knoll at qt.io> wrote:
> Ideally, I would like to get rid of these libraries in src/3rdparty. Instead, we should build up to date versions of these libs in Coin, and simply ship those together with our Qt libraries. With the online installers it should in the longer term even be possible to ship updates for these libraries independent of a new Qt release.

Sounds good to keep them up-to-date.  But if a library makes a source-incompatible change or introduces a bug, it could cause some scrambling on our end to catch up with API changes, or falling back to the last-known-good version.  So, use their HEADs most of the time but also have the ability to freeze for short periods at a specific SHA1? or use actual tagged releases most of the time but also have some way of falling back?