Previously, trying to build from SVN without xsltproc installed would
get you errors like:

  --xinclude -o changes-2.0.1.html changes-2.0.1.xml
  --xinclude:No such file or directory

which is quite confusing.

We now rather just skip the docs completely if we can't find any
xsltproc.  If we actually do need to rebuild the docs (which should
only happen when running make dist), we error out with a useful error
message.



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4330 d4fa192b-c00b-0410-8231-f00ffab90ce4