Refactor: Use gnulib's 'options' module.

Refactor: Use gnulib's 'options' module.

* autogen.sh (GNULIB_MODULES_RUNTIME_FOR_SRC, GNULIB_MODULES_TOOLS_FOR_SRC): Add
'options'. Remove 'getopt-gnu'.
* gettext-runtime/src/envsubst.c: Include options.h instead of <getopt.h>.
(long_options): Remove variable.
(main): Define options array here. Call start_options, get_next_option instead
of getopt_long().
* gettext-runtime/src/gettext.c: Likewise.
* gettext-runtime/src/ngettext.c: Likewise.
* gettext-runtime/src/printf_gettext.c: Likewise.
* gettext-runtime/src/printf_ngettext.c: Likewise.
* gettext-tools/src/cldr-plurals.c: Likewise.
* gettext-tools/src/hostname.c: Likewise.
* gettext-tools/src/msgattrib.c: Likewise.
* gettext-tools/src/msgcat.c: Likewise.
* gettext-tools/src/msgcmp.c: Likewise.
* gettext-tools/src/msgcomm.c: Likewise.
* gettext-tools/src/msgconv.c: Likewise.
* gettext-tools/src/msgen.c: Likewise.
* gettext-tools/src/msgexec.c: Likewise.
* gettext-tools/src/msgfilter.c: Likewise.
* gettext-tools/src/msgfmt.c: Likewise.
* gettext-tools/src/msggrep.c: Likewise.
* gettext-tools/src/msginit.c: Likewise.
* gettext-tools/src/msgmerge.c: Likewise.
* gettext-tools/src/msgunfmt.c: Likewise.
* gettext-tools/src/msguniq.c: Likewise.
* gettext-tools/src/recode-sr-latin.c: Likewise.
* gettext-tools/src/urlget.c: Likewise.
* gettext-tools/src/xgettext.c: Likewise.
* gettext-tools/tests/tstgettext.c: Likewise.
* gettext-tools/tests/tstngettext.c: Likewise.