Generate introspection data

Currently, the introspection data for GLib and its sub-libraries is
generated by gobject-introspection, to avoid the cyclic dependency
between the two projects.

Since gobject-introspection is generally available on installed systems,
we can check for its presence, and generate the introspection data
directly from GLib.

This does introduce a cyclic dependency, which is why it's possible to
build GLib without introspection, then build gobject-introspection, and
finally rebuild GLib.

By having introspection data available during the GLib build, we can do
things like generating documentation; validating newly added API; and
close the loop between adding new API and it becoming available to non-C
consumers of the C ABI (i.e. language bindings).

gobject/meson.build

@@ -163,6 +163,58 @@ pkg.generate(libgobject,
   description : 'GLib Type, Object, Parameter and Signal Library',
 )
 
+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: libglib_dep,
+    extra_args: gir_args + [
+      '-DGLIB_COMPILATION',
+      '-DGETTEXT_PACKAGE="dummy"',
+      '--symbol-prefix=glib',
+      '--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
+
 libgobject_dep = declare_dependency(link_with : libgobject,
   include_directories : [gobjectinc],
   sources : [gobject_visibility_h, glib_enumtypes_h],