[Qt] Build fails with clang
https://bugs.webkit.org/show_bug.cgi?id=115741

Reviewed by Benjamin Poulain.

Since r149112 std::move is used in AtomicString for compilers that
support rvalue references and this requires a standard library
implementing c++11 move semantics.
We only explicitely require c++11 for WebKit2, but since clang supports
rvalue references the build of subtargets using AtomicString fails because
of missing std::move.
It is safe to add CONFIG += c++11 for the whole project when using clang
since we explicitely disable c++11 for ANGLE that had probems with libc++.

* qmake/mkspecs/features/unix/default_pre.prf:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149726 268f45cc-cd09-0410-ab3c-d52691b4dbfc