Sqlite as system library

Sqlite is very configurable. We already make use of it and will embrace it more. Some are simply for performance reasons other are for extensions. So far we already use the JSON extension but we will or want to use the FTS and Spellfix extension, and maybe later the Session extension. We disable the threading safety code too because we do it in our transaction management already. This is advertised for performance reasons. To support system libraries we have to be sure that the system library meets that compilation requirements. Then we want upsert support which is quite recent.

If we want support third parties to enable the system library, how do we want to do it? One possibly would be an environment variable which is documented and is changed with every requirement change. So the user cannot accidentally activate it without checking the documentation.

So far the indexing is disabled for a normal build and will nor be packaged. So this topic has to be decided before we enable the indexing.