Add HAVE_FEATURE_DBCONNECTIVITY to <config_features.h> instead of
using (or not, more likely) -DDISABLE_DBCONNECTIVITY.

The plan is still that database connectivity would be disabled always
for the non-desktop platforms, and otherwise disabling it would be a
configure option.

But, still lots to do; the TiledLibreOffice iOS test app gets tons of
undefineds when linking if I actually try to disable the database
connectivity.

Change-Id: If1e4b55faa1514cd55dec5fc5487a30d689739ac