build: Mandate that sqlite3 >= 3.20.0 has fts5 builtin

That sqlite3 version brought incompatibilities in the fts5 interface,
fts5 modules generated from < 3.20.0 trees won't work with sqlite3
>= 3.20.0, and viceversa.

Since it's not feasible nor desirable to include an additional fts5
module copy nor an embedded sqlite3 amalgamation copy, mandate that
SQLite was compiled with --enable-fts5 if >= 3.20.0 is found.

The current copy of the fts5 module is kept around for sqlite3 < 3.20.0,
so we don't make this a mandatory requirement for earlier versions.

https://bugzilla.gnome.org/show_bug.cgi?id=785883