Compiler warnings with MinGW

Hi all,

Just browsing through the logs of the buildfarm, I have noticed that
some buildfarm animals complain with warnings (jacana uses MinGW):
https://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=jacana&dt=2019-07-19%2001%3A45%3A28&stg=make

There are two of them:
c:/mingw/msys/1.0/home/pgrunner/bf/root/HEAD/pgsql.build/../pgsql/src/backend/port/win32/mingwcompat.c:60:1:
warning: 'RegisterWaitForSingleObject' redeclared without dllimport
attribute: previous dllimport ignored [-Wattributes]

c:/mingw/msys/1.0/home/pgrunner/bf/root/HEAD/pgsql.build/../pgsql/src/bin/pg_basebackup/pg_basebackup.c:1448:8:
warning: variable 'filemode' set but not used
[-Wunused-but-set-variable]
Jul 18 21:59:49     int   filemode;

The first one has been discussed already some time ago and is a cause
of 811be893, but nothing got actually fixed (protagonists in CC):
https://www.postgresql.org/message-id/CABUevEyeZfUvaYMuNop3NyRvvRh2Up2tStK8SXVAPDERf8p9eg@mail.gmail.com

The second one is rather obvious to fix, because we don't care about
the file mode on Windows, so the attached should do the work.  I am
actually surprised that the Visual Studio compilers don't complain
about that, but let's fix it.

Thoughts?
--
Michael