Vcpkg detection is enabled by default, but we did not have a flag to
disable it, and it was not showing up in config.summary either. By
adding a -vcpkg flag, we get to use `-no-vcpkg` when necessary, as well
as adding an entry to config summary indicating whether vcpkg is in use
or not. Besides `-no-vcpkg`, one can pass `-DQT_USE_VCPKG=OFF` to cmake
command in order to disable the automatic vcpkg detection/integration.
[ChangeLog][configure] vcpkg detection, and integration can be disabled
by passing the -no-vcpkg flag to the configure command, or by passing
`-DQT_USE_VCPKG=OFF` to the cmake command.
Pick-to: 6.6
Change-Id: Ide8da70a7b473ec23995104d162356e75e6d1240
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
b3f27f75b6