[gnome-todo] build: Use libadwaita as a submodule

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 70f04fefffdf95fdfe8ea53a9033649fae8b8f8d
Author: Dominique Leuenberger <dimstar opensuse org>
Date:   Sat May 15 00:42:54 2021 +0200

    build: Use libadwaita as a submodule
    
    There have not been any releases of libadwaita yet and it is
    considered unstable.

 .gitmodules            | 3 +++
 meson.build            | 3 ++-
 subprojects/libadwaita | 1 +
 3 files changed, 6 insertions(+), 1 deletion(-)
---
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 00000000..19935b54
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "subprojects/libadwaita"]
+       path = subprojects/libadwaita
+       url = https://gitlab.gnome.org/GNOME/libadwaita
diff --git a/meson.build b/meson.build
index c10efbb8..759b42a6 100644
--- a/meson.build
+++ b/meson.build
@@ -146,11 +146,12 @@ add_project_arguments(common_flags + compiler_flags, language: 'c')
 
 glib_dep = dependency('glib-2.0', version: '>= 2.58.0')
 gtk_dep = dependency('gtk4', version: '>= 3.92.0')
+libadwaita_dep = dependency('libadwaita-1', fallback: ['libadwaita', 'libadwaita_dep'])
 
 gnome_todo_deps = [
   glib_dep,
   gtk_dep,
-  dependency('libadwaita-1'),
+  libadwaita_dep,
   dependency('gio-2.0', version: '>= 2.43.4'),
   dependency('gmodule-export-2.0'),
   dependency('goa-1.0', version: '>= 3.2.0'),
diff --git a/subprojects/libadwaita b/subprojects/libadwaita
new file mode 160000
index 00000000..a1e8587d
--- /dev/null
+++ b/subprojects/libadwaita
@@ -0,0 +1 @@
+Subproject commit a1e8587d623791d3c992a934c5feb46ba209432a