Second try at the change originally made in commit 8518583c;
this time with contrib updates so that manual extern declarations
are also marked with PGDLLEXPORT.  The release notes should point
this out as a significant source-code change for extension authors,
since they'll have to make similar additions to avoid trouble on Windows.

Laurenz Albe, doc change by me

Patch: <A737B7A37273E048B164557ADEF4A58B53962ED8@ntex2010a.host.magwien.gv.at>