Fix #79532: sizeof off_t can be wrong

We have to actually determine the proper `SIZEOF_OFF_T`.
Interestingly, it is `4` on Windows x64.

We also have to prevent the redefinition

Fix #79532: sizeof off_t can be wrong

We have to actually determine the proper `SIZEOF_OFF_T`.
Interestingly, it is `4` on Windows x64.

We also have to prevent the redefinition in pg_config.h. The clean
solution would likely be to not include pg_config.h at all, but that's
out of scope for BC reasons for now.

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