Rename HAS_DOCBOOK option to ENABLE_MAN.
Allow xsltproc to fetch the Docbook stylesheet from the Internet if it's
not installed locally. This will hopefully make it easier for folks
to build the man page.
Previously, git-crypt claimed to support Git as old as 1.6.0 (albeit
with degraded operation). However, this has not been true for some time,
since Git 1.6.0 does not support the --porcelain option to `git status`.
Since Git 1.7.2 was the first version of Git to support filters with
`git blame`, was released almost five years ago (in July 2010), and is
even in Debian Squeeze, it seems like a good minimum version to require.
