C99 compatibility fixes for gnupg 1

[Florian Weimer]

* Werner Koch:

> I applied the patch.  However, changing main to void without having a
> return statement is also prone to errors or warnings, right?

I think reaching the } of main has always been specified to return 0
(except for free-standing environments where main isn't special, but
this doesn't apply here).

And the return type of main does not change whether it is declared
explicitly or not.

Thanks,
Florian