Since we are now building introspection files for GLib while building
GLib, so we want to make sure that we indeed load the freshly-built
DLLs when running g-ir-scanner, so we add the various needed subdirs
(and if needed, subprojects), to set the GI_EXTRA_BASE_DLL_DIRS
envvar so that g-ir-scanner will look for the newly-built GLib DLLs.
Ideally, upstream g-ir-scanner will need to be updated accordingly to do
something similar to what we are doing here, but this is needed until
the time that we require a g-ir-scanner that contains the update.
This will also fix the g-ir-scanner erroring out when there is no
pre-existing GLib on the system, as the needed DLLs are now found.
Related issue: https://gitlab.gnome.org/GNOME/gobject-introspection/-/issues/499
Related MR in G-I:
https://gitlab.gnome.org/GNOME/gobject-introspection/-/merge_requests/458
