libpq++ Makefile uses -DDEBUG to turn on debugging trace to

libpq++ Makefile uses -DDEBUG to turn on debugging trace to
/tmp/trace.out.
However, elog.h uses DEBUG as a log-level flag.  As a result, tracing is
turned on even if the libpq++.so is built with DEBUG commented out in
the Makefile.

This patch changes libpq++ to use DEBUGFILE instead (which is not
defined anywhere else).

Oliver Elphick