clang+libc++ is the only supported way by Google nowadays.
libstdc++ is too old and already fails to build some C++11 apps
e.g. missing std::to_string().
android-g++ mkspec still uses libstdc++ and g++.
Use -isystem to include system headers instead of QMAKE_INCDIR_POST (-I).
Task-number: QTBUG-60455
Change-Id: Iba8b04594c2e5e2832e6cf480e4e52ff31ad4106
Reviewed-by: Sérgio Martins <sergio.martins@kdab.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
