build: stop using libgd.wrap

Try to fix gnome-continuous

This breaks the autotools build but we intend to remove
gedit's autotools support this month.

Closes: #117

.gitmodules

-[submodule "libgd"]
-	path = libgd
-	url = https://gitlab.gnome.org/GNOME/libgd.git
+[submodule "subprojects/libgd"]
+	path = subprojects/libgd
+	url = https://gitlab.gnome.org/gnome/libgd.git

libgd @ 56a19ad9

-Subproject commit 56a19ad9b056332ef07e508f1bdb6f03c6cb2590

meson.build

@@ -48,6 +48,10 @@ localdir = join_paths(datadir, 'local')
 vapidir = join_paths(datadir, 'vala', 'vapi')
 
 # Subprojects
+if not get_option('buildtype').contains('plain')
+  run_command('git', '-C', meson.source_root(), 'submodule', 'update', '--init', '--recursive')
+endif
+
 libgd_sp = subproject(
   'libgd',
   default_options: [

libgd @ c7c7ff4e

+Subproject commit c7c7ff4e05d3fe82854219091cf116cce6b19de0

subprojects/libgd.wrap

-[wrap-git]
-directory=libgd
-url=https://gitlab.gnome.org/GNOME/libgd.git
-revision=head