Upgrade to FTS5

[mjog]

More recent versions of SQLite make it difficult to install custom FTS3/4 tokenisers - 3.11 disables the two-argument form of fts3_tokenizer() needed to install the tokeniser unless built with a compile time flag, while 3.12 only makes it available if preceded by a call to sqlite3_db_config. See https://www.sqlite.org/fts3.html#section_1 for details.

Since Linux distro are starting to ship 3.11, this is causing problems for applications using FTS3/4. So this might be a good time to upgrade to FTS5, which does not have the same problem.