Enabled PDO and PDO_SQLITE by default.
Fixup PDO header detection so that it searches in the correct order, and
correctly picks up the headers when building from outside of the source
tree.
TODO: make pdo_XXX auto-enable when XXX is enabled. Volunteers welcome.
Note that ODBCRouter doesn't directly implement ODBC 3.0, so
using it here won't really work.
Expand the ODBC 3 check to better explain what happened and
how to make progress (e.g.: use an ODBC manager library like
unixODBC).