For top-level builds it's desirable to have "ninja generate_docs" build
all tools needed to generate the documentation.
This is problematic since the doc-generating targets are created before
the doc tool targets. Thus, we must defer the dependency connection if
the doc tool target is not yet available.
This patch adds the functions qt_internal_defer_dependency and
qt_internal_add_deferred_dependencies.
Change-Id: Ica940b80882e67cb0e0943e95541f7f4d1885948
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
