Use C++17 for qmake and force the build of everything with C++17

We will want to use C++17 code in our headers soon.
(including the one in the bootstrap libraries)

This patch is quick and dirty, I guess it will be cleaner once we move to cmake

Updated QMAKE_MACOSX_DEPLOYMENT_TARGET because 10.13 runtime does
not support C++17 stdlib features

Change-Id: I75ac171436945dddd1bb953a9c8d323ac20da7ac
Reviewed-by: Lars Knoll <lars.knoll@qt.io>