build: Move relevant parts to server section.

build: Move relevant parts to server section.

Currently $server_errlist_objs is updated whenever a feature that
affects para_server was detected. This mix of feature detection and
build options is hard to maintain and error-prone, so let's move the
definition of $server_errlist_objs to the already existing section
for para_server.

The patch also kills the unused @server_ldflags@ output variable.