clang: Use __has_feature() to detect C++11 features

clang: Use __has_feature() to detect C++11 features

Apple's clang version is often reported as the next official clang
version, but Apple clang does not support all the features that the
final official clang does. Instead of using version based feature
detection, use the __has_feature() built-in instead, so that we
correctly #define the various Q_COMPILER_* macros.

Task-number: QTBUG-25554
Change-Id: I2195401dbf6cd8574c30aa1bd6f7cf1cf537dae7
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>