Fix "inconsistent DLL linkage" warning on Windows MSVC

This warning was disabled in meson.build (warning 4273).  If you
enable it, it looks like this:

../src/backend/utils/misc/ps_status.c(27): warning C4273: '__p__environ': inconsistent dll linkage
C:\Program Files (x86)\Windows Kits\10\include\10.0.22621.0\ucrt\stdlib.h(1158): note: see previous definition of '__p__environ'

The declaration in ps_status.c was:

    #if !defined(WIN32) || defined(_MSC_VER)
    extern char **environ;
    #endif

The declaration in the OS header file is:

    _DCRTIMP char***    __cdecl __p__environ (void);
    #define _environ  (*__p__environ())

So it is evident that this could be problematic.

The old declaration was required by the old MSVCRT library, but we
don't support that anymore with MSVC.

To fix, disable the re-declaration in ps_status.c, and also in some
other places that use the same code pattern but didn't trigger the
warning.

Then we can also re-enable the warning (delete the disablement in
meson.build).

Reviewed-by: Bryan Green <dbryan.green@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/bf060644-47ff-441b-97cf-c685d0827757@eisentraut.org

src/backend/utils/misc/ps_status.c

@@ -23,7 +23,7 @@
 #include "utils/guc.h"
 #include "utils/ps_status.h"
 
-#if !defined(WIN32) || defined(_MSC_VER)
+#if !defined(WIN32)
 extern char **environ;
 #endif