Update sphinxsearch to 2.1.6.
Fix build on NetBSD. Force libiconv support as standard.

Changes in 2.1.6:
- fixed #1882, race of periodic and forced FLUSHing on an RT index
- fixed #1881, quorum syntax with '.' as blended char
- fixed #1880, crash on multiquery with one incorrect query
- fixed #1876, crash on words with large code points and infix searches
- fixed #1875, fixed crash on adding documents with long words in dict=keyword index with morphology and infixes enabled
- fixed #1857, crash in arabic stemmer
- fixed evaluating of LCS by an expression ranker