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.
This fixes an issue with MSVC where c++latest implied the /std:c++20
compiler flag.
Pick-to: 5.15 6.2 6.3
Fixes: QTBUG-102202
Change-Id: Ie00ee5793c1a649195013c8c19efc8d59cf0acc9
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
b5ed3cb7ba