This is split out from MR !979 (merged) as it looks like this can land first without needing the build support, which will actually need to depend on a fix in Meson, as per @nirbheek's suggestions there. The updates here will make things more usable for clang-cl builds against (real) MSVC builds of the GTK stack using the GLib headers, notably to improve deprecation warning support and to enable g_autoptr support.
The rationale behind this is that
clang-cl will (understandably) define
clang-cl will (understandably) not define
This, as a result, will also make MR !979 (merged) depend on this MR.
With blessings, thank you!