build: Construct server and upgrade_db object list in Makefile.

build: Construct server and upgrade_db object list in Makefile.

Currently configure.ac defines variables for the list of objects of
each executable and exports these via AC_SUBST to Makefile.in. The
lists are then extended and munged further in Makefile.real.

It it easier to construct the lists only in the makefile. So, as
a first step, stop to export the object list of para_server and
para_upgrade_db. Instead, export the variables needed to construct
the list in the makefile.