* Makefile.in ($(srcdir)/po/$(PACKAGE).pot): Pass --keyword=N_ to
`$(XGETTEXT)'.
* include/grub/i18n.h (N_): New macro.
* util/mkisofs/mkisofs.h: Likewise.
* util/mkisofs/mkisofs.c (ld_options): Wrap all translatable strings
around N_().
(usage): Use gettext() to translate help strings when printing them.