obstack: fix license problem

[Simon Josefsson]

Using the obstack module results in warnings:

warning: module obstack depends on a module with an incompatible license: exit
warning: module obstack depends on a module with an incompatible license: 
exitfail

The reason appears to be because obstack is 'LGPL' while exit and
exitfail is 'GPL'.

Bruno, your 'exit' module looks strange: it doesn't do anything except
depending on stdlib.  It used to provide exit.h, but no longer does so.
Can't we simply remove the 'exit' module and suggest users to use the
stdlib module directly instead?

Paul, your exitfail module contains of this effective code:

exitfail.h:
extern int volatile exit_failure;

exitfail.c:

#include <config.h>
#include "exitfail.h"
#include <stdlib.h>
int volatile exit_failure = EXIT_FAILURE;

Would you agree to re-license this module to 'LGPL'?

/Simon