libiberty/setenv.c: Do not declare environ if defined as a macro.

Otherwise, it causes a build warning on some platforms such as MinGW.

libiberty/ChangeLog (Eli Zaretskii  <eliz@gnu.org>):

        * setenv.c <environ>: Declare only if not a macro.

libiberty/setenv.c

@@ -63,8 +63,10 @@ extern int errno;
 
 #define __environ	environ
 #ifndef HAVE_ENVIRON_DECL
+#ifndef environ
 extern char **environ;
 #endif
+#endif
 
 #undef setenv
 #undef unsetenv