Clean up some of the accumulated Autoconf mess here. Put back the

Clean up some of the accumulated Autoconf mess here.  Put back the
PGAC_ARG_ macros so that the options parsing is more robust, put the
macro calls in the right order, and make the detection of the driver
manager behave less oddly.  All the required macros are now in-tree, so
everything can be bootstrapped without external PostgreSQL source trees.