MSYS2 support

There were a number of recent threads about building PostgreSQL on 
MSYS2.  This has been confusing on occasion; see for example [0].  MSYS2 
is actually a derivative of Cygwin.  What most people are actually doing 
is using MSYS2 has the host environment for doing a kind of 
cross-compilation to MinGW.

You can also build natively on MSYS2, using the existing Cygwin support. 
  Except that it won't work because configure doesn't recognize the 
config.guess output.  Attached are a couple of small patches to fix that 
up.  The first patch fixes configure as explained.  The second patch 
fixes some warnings in ps_status.c.  It's curious why the existing build 
farm members don't issue warnings there, but maybe their compilers are 
too old.  The third patch fixes another warning; again, not sure why 
original Cygwin doesn't warn.  It might be a bit too broad to apply like 
that.

MSYS2 doesn't ship with cygserver AFAICT, so you can't run a PostgreSQL 
server, but everything else should work.


[0]: 
https://www.postgresql.org/message-id/6672cebd-0c07-ce1e-36f8-6ae82c496eb0@2ndquadrant.com

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services