Allow installers to specify alternative program names for compression programs.

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.