clang is not considered as a C++11 compiler

By default, clang states it support only C++98 (even clang 5.0)

$ clang++ -dM -E -x c++ /dev/null|grep plusplus
#define __cplusplus 199711L

Clang actually supports more as stated here:

https://clang.llvm.org/cxx_status.html#cxx11

To get the proper define, -std=c++11 should be used

$ clang++ -std=c+11 -dM -E -x c+ /dev/null|grep cplusplus
#define __cplusplus 201103L

A solution could be to change the list https://github.com/qt/qtbase/blob/be93d2de031fb870a1ff244304311e07536a13b9/src/corelib/global/qcompilerdetection.h#L566

by

#  if __cplusplus < 201103L && !(defined(Q_CC_MSVC) && Q_CC_MSVC >= 1800) && !(defined(Q_CC_CLANG) && Q_CC_CLANG >= 303)

but this will still fails on features like atomic

if __has_feature(cxx_atomic) && __has_include(<atomic>)

as this isn't explicitly enable in clang by default.

This is a regression from patch dd59118b87d779e5cbfcd0b4ee2a3d5332433da9