Improve previous changes.

Improve previous changes.

* acinclude.m4: Fix typos.
* gnulib.modules: Add xvasprintf.
* src/common.h: Include xvasprintf.h.
* src/tar.c (options): Remove docstrings for --gzip, --bzip2,
--compress, --lzop, --lzma and --xz.
(tar_help_filter): Generate these using actual values of
*_PROGRAM constants.
(format_default_settings): Use xasprintf.
(parse_opt): Use *_PROGRAM defines instead of hardcoded
program names.