build: Initial attempt at fixing the docs build

We need to reference the types file directly, because it won't be copied
into the builddir by Meson — except for GTK, which needs to generate its
own types file using configure_file().

docs/reference/gdk/gdk4.types

-#include <gdk/gdk.h>
-
 gdk_app_launch_context_get_type
 gdk_cursor_get_type
 gdk_device_get_type

docs/reference/gdk/meson.build

@@ -156,10 +156,9 @@ gnome.gtkdoc('gdk4',
 	       join_paths(meson.build_root(), 'gdk'),
 	     ],
              dependencies: libgtk_dep,
-             gobject_typesfile: 'gdk4.types',
+             gobject_typesfile: join_paths(meson.source_root(), 'docs/reference/gdk/gdk4.types'),
              scan_args: [
-               '--ignore-decorators=_GDK_EXTERN',
-	       '--ignore-decorators=G_GNUC_WARN_UNUSED_RESULT',
+               '--ignore-decorators=_GDK_EXTERN|G_GNUC_WARN_UNUSED_RESULT',
                '--ignore-headers=' + ' '.join(private_headers),
              ],
              fixxref_args: [

docs/reference/gsk/meson.build

@@ -43,10 +43,9 @@ gnome.gtkdoc('gsk4',
 	       join_paths(meson.build_root(), 'gsk'),
 	     ],
              dependencies: libgtk_dep,
-             gobject_typesfile: 'gsk4.types',
+             gobject_typesfile: join_paths(meson.source_root(), 'docs/reference/gsk/gsk4.types'),
              scan_args: [
                '--ignore-decorators=_GDK_EXTERN',
-	       '--ignore-decorators=G_GNUC_WARN_UNUSED_RESULT',
                '--ignore-headers=' + ' '.join(private_headers),
              ],
              fixxref_args: [

docs/reference/gtk/meson.build

@@ -6,6 +6,21 @@ images = [
 
 configure_file(input: 'version.xml.in', output: 'version.xml', configuration: version_conf)
 
+types_conf = configuration_data()
+if os_win32
+  types_conf.set('DISABLE_ON_W32', '%')
+else
+  types_conf.set('DISABLE_ON_W32', '')
+endif
+
+if os_darwin
+  types_conf.set('DISABLE_ON_QUARTZ', '%')
+else
+  types_conf.set('DISABLE_ON_QUARTZ', '')
+endif
+
+configure_file(input: 'gtk4.types.in', output: 'gtk4.types', configuration: types_conf)
+
 gnome.gtkdoc('gtk4',
              main_xml: 'gtk4-docs.xml',
              src_dir: [
@@ -15,8 +30,7 @@ gnome.gtkdoc('gtk4',
              dependencies: libgtk_dep,
              gobject_typesfile: 'gtk4.types',
              scan_args: [
-               '--ignore-decorators=_GDK_EXTERN',
-	       '--ignore-decorators=G_GNUC_WARN_UNUSED_RESULT',
+               '--ignore-decorators=_GDK_EXTERN|G_GNUC_WARN_UNUSED_RESULT',
                '--ignore-headers=' + ' '.join(private_headers),
              ],
              fixxref_args: [