Add the CONFIG value c++2b that represent the upcoming C++23 standard.
Add QMAKE_CXXFLAGS_CXX2B and QMAKE_CXXFLAGS_GNUCXX2B.
On MSVC, use /std:c++latest when c++2b is active.
Task-number: QTBUG-102202
Change-Id: Ie00ee5793c1a649195013c8c19efc8d59cf0acc9
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
(cherry picked from commit b5ed3cb7ba)
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>