We've been burnt by this issue repeatedly (cf c2d1eea9e, d025cf88b,
11b500072) so I think it's time to try to formalize and document
what to do to export a variable from src/common/ or src/port/.
Here's a draft patch. I'm not in love with the name "PGDLLIMPORT_FE"
and would welcome better ideas.
regards, tom lane