meson: Require gi-docgen when gtk_doc is required
Fixes build with only -Dgtk_doc=true
without
-Dintrospection=enabled
:
Program gi-docgen found: NO
Configuring gdk4.toml using configuration
docs/reference/gdk/meson.build:13:2: ERROR: Tried to use not-found external program in "command"
introspection
is auto
by default.
The rest of the docs build is only contingent on
if get_option('gtk_doc')
so we should use the same restriction here.
We check for build_gir
below already, and the gi-docgen subproject
itself does not need gobject-introspection so it's fine to do it like
this.