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