Add an abstraction over Qt::WinMain (aka qtmain.lib) and
iOS's runtime linker entry point (_qt_main_wrapper).
The Core target will now link against the Startup target on all
platforms, instead of just WinMain on Windows.
The creation and linkage interface definition of the Startup target
is done at find_package(Qt6Core) time via the private call of
_qt_internal_setup_startup_target().
This will add automatic linkage of WinMain to executables marked with
the WIN32_EXECUTABLE property on Windows.
As well as the addition of the '-Wl,-e,_qt_main_wrapper' linker flag
when linking iOS executables.
Qt users can opt out of this behavior by either setting the
QT_NO_LINK_QTMAIN property or variable. This is in line with
Qt 5 behavior.
Task-number: QTBUG-87060
Change-Id: I7d5e9f1be0e402cf8e67e6f55bfd285f9e6b04f4
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
