>> I did make distclean, and I retried "configuration and ", but I still
>> saw the following errors:
>>
>
> The only other theory I can think of is that the linker is picking up
> an old libpq.so from /home/postgresql/lib because of the -rpath switch
> ... which it should not be doing, but maybe it is anyway. If there's
> an 8.0 libpq.so there, try getting rid of it.
You are absolutely correct. It is because of the libpg.so.
I used the central gcc before, and -rpath was linked to the 8.0 libpg.so.
Now I specified the local gcc and installed version 8.1.4 successfully
this time.
Thanks a lot for all your kind help!