Use of PRId64 with PostgreSQL functions

I've been currently suffering an issue in mingw64 with changes made in
PostGIS topology code base.  Sadly it only happens to me (compiling in
mingw64)

Issue is when code like this was introduced

 appendStringInfo(str, "%s%" PRId64, sep, edge->start_node);


Which uses a PostgreSQL function appendStringInfo - as noted here:
https://trac.osgeo.org/postgis/ticket/3206

AFAICS -- the printf function works fine with the PRId64 so guessing it must
be something special about appenStringInfo


I noticed a thread in March on pgsql-hackers that discussed int limits 

http://www.postgresql.org/message-id/20150331141423.GK4878@alap3.anarazel.de


in PostgreSQL and Andres  Freund made this comment which stuck in my head:

" for another we'd need some uglyness to determine the
correct printf modifier for int64_t (can't use PRId64 etc afaics)."


What exactly did he mean by can't use PRId64?


Thanks,
Regina Obe