build: Move gir generation to an introspection folder

Generating gir and typelib files has inter-dependencies that may depend
on other elements.

For example, glib requires gobject and gdump generated files require
gmodule, so we've a cyclic dependency because gmodule requires gobject,
that requires glib.

To prevent this, let's just generate the introspection files at once in
a different meson file so that we don't have to deal with this.

As per this we could even revert commit fa37ab6d0 since gio is now
compiled before the gir files.

gobject/meson.build

@@ -170,59 +170,6 @@ libgobject_dep = declare_dependency(link_with : libgobject,
 )
 meson.override_dependency('gobject-2.0', libgobject_dep)
 
-if enable_gir
-  # We generate GLib-2.0 here, as we need various GTypes
-  glib_gir = gnome.generate_gir(libglib,
-    sources: [
-      glibconfig_h,
-      gversionmacros_h,
-      glib_visibility_h,
-      glib_headers,
-      glib_deprecated_headers,
-      glib_sub_headers,
-      glib_enumtypes_h,
-      glib_types_h,
-      glib_deprecated_sources,
-      glib_sources,
-    ],
-    namespace: 'GLib',
-    nsversion: '2.0',
-    identifier_prefix: 'G',
-    symbol_prefix: 'g',
-    export_packages: 'glib-2.0',
-    header: 'glib.h',
-    install: true,
-    dependencies: libgobject_dep,
-    extra_args: gir_args + [
-      '-DGLIB_COMPILATION',
-      '-DGETTEXT_PACKAGE="dummy"',
-      '--symbol-prefix=glib',
-      '--library-path=' + meson.current_build_dir(),
-      '--library=gobject-2.0',
-    ],
-  )
-
-  gobject_gir = gnome.generate_gir(libgobject,
-    sources: [
-      gobject_visibility_h,
-      gobject_install_headers,
-      gobject_sources,
-    ],
-    namespace: 'GObject',
-    nsversion: '2.0',
-    identifier_prefix: 'G',
-    symbol_prefix: 'g',
-    export_packages: 'gobject-2.0',
-    header: 'glib-object.h',
-    includes: [ glib_gir[0] ],
-    install: true,
-    extra_args: gir_args + [
-      '-DGOBJECT_COMPILATION',
-      '--symbol-prefix=gobject',
-    ],
-  )
-endif
-
 gobject_query = executable('gobject-query', 'gobject-query.c',
   install : true,
   install_tag : 'bin-devel',