msys2 is missing pexports

We rely on pexports to extract exported symbols from DLL files (e.g. for
linking in PLs) when building with mingw. However, this program isn't
present in msys2. Instead the approved way is apparently to call
"gendef" from the appropriate toolset (e.g. /mingw64/bin). I have worked
around this on my new test machine but adding a script liker this in
/usr/bin/pexports:

#!/bin/sh
gendef - "$@"

However, requiring that is a bit ugly. Instead I think we should do
something like the attached.

I would not be surprised if we need to test the msys version elsewhere
as time goes on, so this would stand us in good stead if we do.

cheers

andrew

--
Andrew Dunstan https://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services