Update after gnulib changed: Support building with --enable-threads=isoc.
* gettext-runtime/intl/dcigettext.c (lock): Move outside _nl_find_msg.
* gettext-runtime/intl/finddomain.c (lock): Move outside _nl_find_domain.
* gettext-runtime/intl/loadmsgcat.c (lock): Move outside _nl_load_domain.
