build: Tweak gdk-pixbuf subproject

Use builtin loaders by default, so we don't make everybody figure
out how to make gdk-pixbuf find uninstalled loaders.

meson.build

@@ -435,7 +435,7 @@ endif
 
 pangocairo_dep    = dependency('pangocairo', version: pango_req)
 pixbuf_dep        = dependency('gdk-pixbuf-2.0', version: gdk_pixbuf_req,
-                               default_options: ['png=enabled', 'jpeg=enabled', 'builtin_loaders=png,jpeg', 'man=false'])
+                               default_options: ['png=enabled', 'jpeg=enabled', 'gif=enabled', 'builtin_loaders=all', 'man=false'])
 png_dep           = dependency('libpng', 'png')
 tiff_dep          = dependency('libtiff-4', 'tiff')
 jpeg_dep          = dependency('libjpeg', 'jpeg')

subprojects/gi-docgen.wrap

@@ -7,3 +7,4 @@ depth = 1
 
 [provide]
 program_names = gi-docgen
+gi-docgen = dummy_dep