DLLIMPORT definition

The DLLIMPORT definition used on Win32 conflicts with the headers in TCL,
at least, and possibly others.

One way to fix it is similar to the HAVE_xyz ones that I talk about in my
other email. Another way to do it would be for us to use PGDLLIMPORT
instead of DLLIMPORT. That way we'd be sure not to conflict with any
*other* third party modules as well, which could happen if you have a
server-side module that links pg to something.

We seem to have a little over 100 entries of DLLIMPORT including comments
and the ecpg regression tests that duplicate it in the output files,
so it's not a huge thing to change.

Thoughts?

//Magnus