Compression and Checksums

[Anders F Björklund]

The "use_xz" directive was added on trunk, just like
the previous "use_lzma" and "use_bzip2" already in.
It will use a distfile with .xar.xz instead of tar.gz,
and needs the XZ Utils installed to decompress that.

Since before, you can also use both xz and lzma to
make package archives. This is done by changing the
portarchivetype from the default "tgz" to use "txz",
or you can use more than one (to compare the sizes)

portarchivemode		yes
portarchivetype		tgz,txz


The "sha256" digest was also added to Pextlib, as
a future alternative to the SHA-1/RMD-160 combo.
It's longer than those (32 instead of 20), but
smaller than needing to have both of them (=40).

It's not yet available for the portfiles, until a
policy decision has been made on which checksums
to use for distfiles. Additionally "size" might be
useful to have, instead of distcheck.check=filesize ?

# The list of the types of checksums we know.
set checksum_types "md5 sha1 rmd160"

--anders