If AM_GNU_GETTEXT is seen in configure.ac, then Automake
turns on support for GNU gettext, a message catalog system for
internationalization
(see Introduction in GNU gettext utilities).
The gettext support in Automake requires the addition of one or
two subdirectories to the package: po and possibly also intl.
The latter is needed if AM_GNU_GETTEXT is not invoked with the
‘external’ argument, or if AM_GNU_GETTEXT_INTL_SUBDIR is used.
Automake ensures that these directories exist and are mentioned in
SUBDIRS.