intl: Assume strdup exists.
* gettext-runtime/intl/bindtextdom.c (set_binding_values): Assume HAVE_STRDUP
to be 1.
* gettext-runtime/intl/finddomain.c (_nl_find_domain): Likewise.
* gettext-runtime/intl/textdomain.c (TEXTDOMAIN): Likewise.
* gettext-runtime/m4/intl.m4 (gt_INTL_SUBDIR_CORE): Don't test for strdup.
