Add the QT_USE_VCPKG variable and the VCPKG_ROOT environment variable handling to the Qt toolchain file. The below rules applicable when configuring Qt project or Qt repository using the qt.toolchain.cmake file: If qtbase(or top-level qt) was initialy configured with vcpkg the new code path will be ignored, if the QT_CHAINLOAD_TOOLCHAIN_FILE matches the path to the vcpkg toolchain file. If qtbase(or top-level qt) was configured with some other toolchain file provided it will be chainloaded next to the vcpkg one using VCPKG_CHAINLOAD_TOOLCHAIN_FILE variable, and the vcpkg toolchain file will be chainloaded by Qt toolchain using QT_CHAINLOAD_TOOLCHAIN_FILE code path. If qtbase(or top-level qt) was configured without extra toolchain file specified, the vcpkg toolchain file will be chainloaded by Qt toolchain using QT_CHAINLOAD_TOOLCHAIN_FILE code path. So the generic toolchain file load sequence can be described as following: qt.toolchain.cmake[->vcpkg.cmake][->customtoolchain.cmake] The QT_USE_VCPKG value is inherited from qtbase(or top-level qt) build by default. We also preserve the VCPKG_TARGET_TRIPLET, but allow using the one that is specified by user. Change-Id: I85c8ed4e9abd658609ac6ea71393dd23a9bc1688 Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>