MinGW compilation warnings in libiberty's include/environ.h

[Eli Zaretskii <eliz@gnu.org>]

When compiling libiberty (as part of GDB) with MinGW on MS-Windows, I
see the following warning:

     gcc -c -DHAVE_CONFIG_H -O2 -gdwarf-4 -g3 -D__USE_MINGW_ACCESS  -I. -I./../include   -W -Wall -Wwrite-strings -Wc++-compat -Wstrict-prototypes -pedantic  -D_GNU_SOURCE ./setenv.c -o setenv.o
     In file included from ./setenv.c:64:0:
     ./../include/environ.h:30:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
      extern char **environ;
      ^

This was already reported 4 years ago, here:

  https://gcc.gnu.org/ml/gcc-patches/2013-03/msg00471.html

and was solved back then.  But it looks like the offending code was
copied to include/environ.h without the fix, and the warning is thus
back.

The problem is with this code in environ.h:

  #ifndef HAVE_ENVIRON_DECL
  #  ifdef __APPLE__
  #     include <crt_externs.h>
  #     define environ (*_NSGetEnviron ())
  #  else
  extern char **environ;
  #  endif
  #  define HAVE_ENVIRON_DECL
  #endif

which causes the MinGW compiler to see the declaration of environ,
whereas MinGW's stdlib.h has this:

  #ifdef __MSVCRT__
  # define _environ  (*__p__environ())
  extern _CRTIMP __cdecl __MINGW_NOTHROW  char ***__p__environ(void);
  # define _wenviron  (*__p__wenviron())
  extern _CRTIMP __cdecl __MINGW_NOTHROW  wchar_t ***__p__wenviron(void);

  #else  /* ! __MSVCRT__ */
  # ifndef __DECLSPEC_SUPPORTED
  # define _environ (*_imp___environ_dll)
  extern char ***_imp___environ_dll;

  # else  /* __DECLSPEC_SUPPORTED */
  # define _environ  _environ_dll
  __MINGW_IMPORT char ** _environ_dll;
  # endif  /* __DECLSPEC_SUPPORTED */
  #endif  /* ! __MSVCRT__ */

  #define environ _environ

Can this be fixed again, please?  The solution, as back then, is this:

  #ifndef HAVE_ENVIRON_DECL
  #  ifdef __APPLE__
  #     include <crt_externs.h>
  #     define environ (*_NSGetEnviron ())
  #  else
  #     ifndef environ
  extern char **environ;
  #     endif
  #  endif
  #  define HAVE_ENVIRON_DECL
  #endif

Thanks.