meson: update submodule only if it doesn’t exist

Some build servers don’t have git installed, which makes the tarball
build fail when trying to update the libgd submodule. This commit adds a
check for the existence of the root meson.build of libgd before running
git.

https://bugzilla.gnome.org/show_bug.cgi?id=779603

check_libgd.sh

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

meson.build

@@ -102,8 +102,8 @@ nautilus_include_dirs = include_directories ('.')
 subdir ('eel')
 subdir ('libnautilus-extension')
 
-message ('Updating libgd submodule…')
-run_command ('git', '--git-dir="$MESON_SOURCE_ROOT/.git"', 'submodule', 'update', '--init', 'libgd')
+message ('Checking for libgd…')
+run_command ('check_libgd.sh')
 
 libgd_options = ['static=true',
                  'with-gtk-hacks=true',