our binary packages come without many sql drivers, because they have
proprietary dependencies we cannot ship. not every user wants to build
all of qt from scratch, so it makes sense to make it possible to
"enrich" the existing installation by compiling just the drivers. to
enable this, the drivers' configuration must be independent. but note
that it's still not possible to configure a single driver - the entire
sqldrivers directory is configured at once.
a side effect of this is that the availability of the sql plugins
cannot be made known with publicFeatures any more, because there is no
associated module pri file to put that information into. that should be
made inconsequential by making qtHaveModule() work for plugins.
Reviewed-by: Joerg Bornemann <email@example.com>