utils: Use nominative/genitive month names correctly

These changes are needed since the standalone (nominative) vs. full date
format (genitive) month names support has been introduced in Linux with
glibc 2.27.  They are also supported in *BSD systems.

See also:
https://bugzilla.gnome.org/show_bug.cgi?id=749206
https://sourceware.org/bugzilla/show_bug.cgi?id=10871

https://gitlab.gnome.org/GNOME/gnome-calendar/issues/125

src/gcal-date-chooser.c

@@ -224,20 +224,6 @@ calendar_get_weekday_name (gint i)
   return text;
 }
 
-/* 0 = january */
-static gchar *
-calendar_get_month_name (gint i)
-{
-  GDateTime *date;
-  gchar *text;
-
-  date = g_date_time_new_local (2015, i + 1, 1, 1, 1, 1);
-  text = g_date_time_format (date, "%B");
-  g_date_time_unref (date);
-
-  return text;
-}
-
 static void
 calendar_init_weekday_display (GcalDateChooser *self)
 {
@@ -257,7 +243,7 @@ format_month (GcalMultiChoice *choice,
               gint             value,
               gpointer         data)
 {
-  return calendar_get_month_name (value);
+  return g_strdup (gcal_get_month_name (value));
 }
 
 static void
@@ -269,9 +255,8 @@ calendar_init_month_display (GcalDateChooser *self)
 
   for (i = 0; i < 12; i++)
     {
-      month = calendar_get_month_name (i);
-      months[i] = g_strdup_printf ("%s", month);
-      g_free (month);
+      month = gcal_get_month_name (i);
+      months[i] = g_strdup (month);
     }
 
   months[12] = NULL;