[Development] buildsystem now needs to keep behaviour compatibility

[Thiago Macieira]

[This is mostly to Ossi, Jörg and myself, but sending to everyone FYI]

Now that we will no longer include qtwebkit, qtquick1 and qtscript in the main 
releases, we are asking people to continue building the last released version 
of some of those modules against newer Qt versions. That was in the 5.5.0 
changelog:

 - The QtWebKit and QtQuick1 modules and support for the QML 1 language
   are deprecated and Qt 5.5 will be the last release to include
   them. Starting with Qt 5.6, the source code for those modules will
   not be included in Qt's packaging. Compiling the 5.5 release of
   QtWebKit modules along with Qt 5.6 or future versions should
   work. QtQuick1 is not guaranteed to work in future versions after
   Qt 5.6.

 - The QtScript module is deprecated and will be removed from Qt's
   packaging starting with version 5.7. The 5.5 and 5.6 releases of
   QtScript should continue to work along with Qt 5.7 and future
   versions.

Therefore, whenever you're making changes to the buildsystem, please test the 
last releases of qtwebkit and qtscript to verify that they don't break the 
build. If they do, your change is not allowed and you should redesign 
(probably by making your change an opt-in for the other modules).

I'm guilty and will need to rework one change I made.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center