Note that some tests now fail; if we can't resolve this in time for the beta,
the prepare code should be disabled (I'll add a flag for this later today).
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.
