Pull requests: CJK words and Snippet generator

[Robert Stepanek]

Thanks to Olly for the enhanced snippet generator! I've closed the pull
request for the term cover snippet patch and opened a tiny follow-up
pull request: For Cyrus, we expect MSet::snippet to return an empty
string if not a single search term was found in the text, and I've added
an optional flag for that. The existing behaviour to return a substring
of the input without markup remains the default.
https://github.com/xapian/xapian/pull/130

The CJK word boundary analysis pull request has been rebased against
latest master. We'd be very happy to see this merged into upstream, it
has served us well since a couple of months, already. The PR make libicu
a mandatory dependency and keeping it at that would make the patch
straightforward. If it's necessary, I'll see if I can make ICU optional
and set the according #ifdefs in the code. See
https://github.com/xapian/xapian/pull/114

Unfortunately, both pull requests currently do not pass Travis. The
build breaks during linking in xapian-letor with the message: 

    bin/xapian-prepare-trainingfile.o: undefined reference to symbol
    '_ZNK6Xapian5Error15get_descriptionEv'

Is that a known issue? Has anyone experienced the same? E.g. see
https://travis-ci.org/xapian/xapian/jobs/190191549#L3541
 
Cheers,
Robert