environ: Fix wrong autoconf test result in C++ mode

[Bruno Haible]

The autoconf test
  checking if environ is properly declared... 
produces a wrong result (yes instead of no) in C++ mode, with g++ version 6
or newer and with clang++.

The reason is this error message:
g++:
  error: '<anonymous struct> environ', declared using anonymous type, is used 
but never defined
clang++:
  error: variable 'environ' is used but not defined in this translation unit, 
and cannot be defined in any other translation unit because its type does not 
have linkage

This patch fixes it.


2021-06-27  Bruno Haible  <bruno@clisp.org>

        environ: Fix wrong autoconf test result in C++ mode.
        * m4/environ.m4 (gt_CHECK_VAR_DECL): Use a typedef'ed type, not an
        anonymous type.

diff --git a/m4/environ.m4 b/m4/environ.m4
index d971770..ae53291 100644
--- a/m4/environ.m4
+++ b/m4/environ.m4
@@ -1,4 +1,4 @@
-# environ.m4 serial 7
+# environ.m4 serial 8
 dnl Copyright (C) 2001-2004, 2006-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -33,7 +33,8 @@ AC_DEFUN([gt_CHECK_VAR_DECL],
     [AC_COMPILE_IFELSE(
        [AC_LANG_PROGRAM(
           [[$1
-            extern struct { int foo; } $2;]],
+            typedef struct { int foo; } foo_t;
+            extern foo_t $2;]],
           [[$2.foo = 1;]])],
        [gt_cv_var=no],
        [gt_cv_var=yes])])