uri: Don't rely on g_time_zone_new_offset()

g_time_zone_new_offset() was new in GLib 2.58, but Ubuntu 18.04 'bionic'
only has GLib 2.56, and in theory we still claim to support versions
all the way back to GLib 2.46. If that function isn't available,
reimplement it in terms of the deprecated g_time_zone_new().

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 3591ba08f6)

common/flatpak-uri.c

@@ -1614,6 +1614,41 @@ parse_time (int *hour, int *minute, int *second, const char **date_string)
   return TRUE;
 }
 
+static inline GTimeZone *
+time_zone_new_offset (gint32 offset)
+{
+#if GLIB_CHECK_VERSION (2, 58, 0)
+  return g_time_zone_new_offset (offset);
+#else
+  g_autofree char *id = NULL;
+  gint hours, minutes;
+  gint seconds = offset;
+  GTimeZone *tz;
+  char sign = '+';
+
+  if (seconds == 0)
+    return g_time_zone_new_utc ();
+
+  if (seconds < 0)
+    {
+      seconds = -seconds;
+      sign = '-';
+    }
+
+  hours = seconds / 3600;
+  seconds = seconds % 3600;
+  minutes = seconds / 60;
+  seconds = seconds % 60;
+
+  id = g_strdup_printf ("%c%02d:%02d:%02d", sign, hours, minutes, seconds);
+  tz = g_time_zone_new (id);
+  /* If this assertion fails, we'll log a critical but still return tz,
+   * which is documented to be UTC if the time zone could not be parsed */
+  g_return_val_if_fail (g_time_zone_get_offset (tz, 0) == offset, tz);
+  return tz;
+#endif
+}
+
 static inline gboolean
 parse_timezone (GTimeZone **timezone_out, const char **date_string)
 {
@@ -1664,7 +1699,7 @@ parse_timezone (GTimeZone **timezone_out, const char **date_string)
   if (utc)
     *timezone_out = g_time_zone_new_utc ();
   else
-    *timezone_out = g_time_zone_new_offset (offset_minutes * 60);
+    *timezone_out = time_zone_new_offset (offset_minutes * 60);
 
   return TRUE;
 }