[Development] Dropping qtserialbus from Qt 5.6 -- it doesn't compile in C++98

[Blasche Alexander]

>Because you also need to reject some older Clang versions. Did you add the
>check for them too?

clang 3.4 is rejected by requires(c++11) already.

>Please think of Clang on Linux and FreeBSD, plus the older XCode that we still
>support on OS X. This is LTS and the last version before we require C++11, so
>expect people will try to build it with older versions.

>Why do we insist on testing for something that proxies the real need? We need
>X and we know that Y provides X, so we test for Y. Why can't we just test for
>X?

Effort and speed. The compile test should not delay the 5.6.0 release and such a test would take much more time. After all we would have to go through the entire code base and identify all C++11 features we are using. That's bound to have the same gaps too. 

I want to emphasize qtserialbus is not the first module to do such a compiler based rejection pattern. For 5.6.0 this is good enough.

Nevertheless I filed https://bugreports.qt.io/browse/QTBUG-51655 and we might have a better solution for 5.6.1.

--
Alex