* 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.
This adds --with-gzip, --with-bzip2 etc. switches to the configure, so that
one can do, e.g. ./configure --with-bzip2=lbzip2 and have lbzip2 executed
whenever user calls `tar --bzip2'.
* acinclude.m4: New file.
* configure.ac: Add TAR_COMPR_PROGRAM invocations for
the supported compressors.
* src/buffer.c (magic): Use *_COMPRESSOR defines instead
of hardcoded program names.
* src/suffix.c (compression_suffixes): Likewise.
