Fixed parsing problem with multiple inclusions of alt typedefs.

<b>Multiple passes on type definitions with /*@alt@*/</b>
<blockquote>
Splint will produce a parse error if a type definition like,
<pre>
typedef long /*@alt int@*/ newtype;
</pre>
is processed twice (for example if it is in a header file included
multiple times).
<p>
Reorted by Simon Hosie, fixed in CVS (2002-12-12).
</blockquote>