meson: delegate submodule initialization to Meson

As of 0.40.0, Meson has the ability to initialize subprojects that are
git submodules. This removes the need for a separate script to achieve
that.

https://bugzilla.gnome.org/show_bug.cgi?id=781653

build-aux/meson/check_libgd.sh

@@ -1,8 +0,0 @@
-#!/bin/sh
-
-if [ ! -f "${MESON_SOURCE_ROOT}/subprojects/libgd/meson.build" ]
-then
-    git \
-        --git-dir="${MESON_SOURCE_ROOT}/.git" \
-        submodule update --init subprojects/libgd
-fi

meson.build

@@ -1,6 +1,6 @@
 project ('nautilus', 'c',
          version: '3.23.91',
-         meson_version: '>=0.37.0',
+         meson_version: '>=0.40.0',
          license: 'GPL2+')
 
 add_global_arguments ('-Wno-deprecated-declarations', language:'c')
@@ -100,9 +100,6 @@ nautilus_include_dirs = include_directories ('.')
 subdir ('eel')
 subdir ('libnautilus-extension')
 
-message ('Checking for libgd…')
-run_command ('build-aux/meson/check_libgd.sh')
-
 libgd_options = ['static=true',
                  'with-gtk-hacks=true',
                  'with-main-view=true',