New file, from gnulib.

m4/setenv.m4

@@ -0,0 +1,72 @@
+# setenv.m4 serial 5
+dnl Copyright (C) 2001-2004 Free Software Foundation, Inc.
+dnl This file is free software, distributed under the terms of the GNU
+dnl General Public License.  As a special exception to the GNU General
+dnl Public License, this file may be distributed as part of a program
+dnl that contains a configuration script generated by Autoconf, under
+dnl the same distribution terms as the rest of that program.
+
+AC_DEFUN([gt_FUNC_SETENV],
+[
+  AC_REPLACE_FUNCS(setenv unsetenv)
+  if test $ac_cv_func_setenv = no; then
+    gl_PREREQ_SETENV
+  fi
+  if test $ac_cv_func_unsetenv = no; then
+    gl_PREREQ_UNSETENV
+  else
+    AC_CACHE_CHECK([for unsetenv() return type], gt_cv_func_unsetenv_ret,
+      [AC_TRY_COMPILE([#include <stdlib.h>
+extern
+#ifdef __cplusplus
+"C"
+#endif
+#if defined(__STDC__) || defined(__cplusplus)
+int unsetenv (const char *name);
+#else
+int unsetenv();
+#endif
+], , gt_cv_func_unsetenv_ret='int', gt_cv_func_unsetenv_ret='void')])
+    if test $gt_cv_func_unsetenv_ret = 'void'; then
+      AC_DEFINE(VOID_UNSETENV, 1, [Define if unsetenv() returns void, not int.])
+    fi
+  fi
+])
+
+# Check if a variable is properly declared.
+# gt_CHECK_VAR_DECL(includes,variable)
+AC_DEFUN([gt_CHECK_VAR_DECL],
+[
+  define([gt_cv_var], [gt_cv_var_]$2[_declaration])
+  AC_MSG_CHECKING([if $2 is properly declared])
+  AC_CACHE_VAL(gt_cv_var, [
+    AC_TRY_COMPILE([$1
+      extern struct { int foo; } $2;],
+      [$2.foo = 1;],
+      gt_cv_var=no,
+      gt_cv_var=yes)])
+  AC_MSG_RESULT($gt_cv_var)
+  if test $gt_cv_var = yes; then
+    AC_DEFINE([HAVE_]translit($2, [a-z], [A-Z])[_DECL], 1,
+              [Define if you have the declaration of $2.])
+  fi
+])
+
+# Prerequisites of lib/setenv.c.
+AC_DEFUN([gl_PREREQ_SETENV],
+[
+  AC_REQUIRE([AC_FUNC_ALLOCA])
+  AC_CHECK_HEADERS_ONCE(unistd.h)
+  AC_CHECK_HEADERS(search.h)
+  AC_CHECK_FUNCS(tsearch)
+  gt_CHECK_VAR_DECL([#include <errno.h>], errno)
+  gt_CHECK_VAR_DECL([#include <unistd.h>], environ)
+])
+
+# Prerequisites of lib/unsetenv.c.
+AC_DEFUN([gl_PREREQ_UNSETENV],
+[
+  AC_CHECK_HEADERS_ONCE(unistd.h)
+  gt_CHECK_VAR_DECL([#include <errno.h>], errno)
+  gt_CHECK_VAR_DECL([#include <unistd.h>], environ)
+])