conan / macos sysroot not taken into account

the `auto-setup.cmake` doesn't take the `CMAKE_OSX_SYSROOT`. this causes e.g. the cityhash recipe to fail building.

if i explicitly use `conan_provider.cmake`, the builds succeed

-DCMAKE_PROJECT_TOP_LEVEL_INCLUDES=cmake/conan_provider.cmake
-DQT_CREATOR_SKIP_CONAN_SETUP=True

reason for this is that the sysroot needs to be set in order to resolve certain system libraries (otherwise a simple main.c fails to link with `ld: library 'System' not found`)