Minimal portability fix for commit
e1551f96e.
Older gcc versions are not happy with having multiple declarations
for the same typedef name (not struct name). I'm a bit dubious
as to how well-thought-out that patch was at all, but for the moment
just fix it enough so I can get some work done today.
Discussion: https://postgr.es/m/
20191218101338.GB325369@paquier.xyz