[Development] Removing the -c++11 option from configure

[Thiago Macieira]

We'd like to make Qt build unconditionally with the latest version of the C++ 
standard that is supported by the compiler. That implies removing the -c++11 
option so that the -no-c++11 option goes away too.

Possible drawbacks:
 - Inability to test non-C++11 codepaths in the CI
   => We can add a hidden option for the CI

 - OS X support without libc++ may go away
   => I need more information on this

Advantage:
 - Will turn on C++14 for recent GCC and Clang

Anything I missed? Opinions?

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