install libpq.dll in bin directory on Windows / Cygwin

Jeff Janes asked me about this, and Bruce just tripped up on it. Usually
on Windows it's necessary to have libpq.dll/cygpq.dll either in the PATH
or in the same directory as client .exe files. The buildfarm client has
for many years simply copied this dll from the installation lib to the
installation bin directory after running "make install". But I can't
really see why we don't do that as part of "make install" anyway. I
haven't tested but I think something like this patch would achieve this
goal - it would fix something that's tripped a lot of people up over the
years.

Comments? If we do this, should it be backported?

cheers

andrew