* If GETTEXT_PACKAGE is not defined (ex. by build system/user) then
  define it to NULL. Most GLib gettext-functions allow passing NULL
  to use the domain set with textdomain().
* Improve the ngettext() wrapper macro.
* Improve/fix bind_textdomain_codeset() wrapper macro.
* Fix argument in bindtextdomain()
* Cast bind*textdomain*() results with (void) to get rid of compiler
  warnings when building without GETTEXT_PACKAGE defined.