libpq/Makefile cleanup abandoned

I thought I could improve the way libpq/Makefile pulls in C files, but
it turns out MSVC scrapes the OBJS lines from that file, so I had to
revert most of my cleanup, attached.

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +