Make h2ph generate constant subs

Make h2ph generate constant subs

>> Shouldn't these:
>>     eval 'sub RUN_LVL {1;}';
>> be:
>>     eval 'sub RUN_LVL() {1;}';
>
> I'd say so.  Who can tell us whether h2ph is easily changeable?

It is, here's a patch.  It could still use changes so that it emits new
style sub invocations so that more constant folding happens, but that's
trickier.  Additional changes in the patch are removal of a "chdir
/usr/include" (which prevented you from converting files in other
directories and wasn't needed for the standard invocation) and addition
of a -d switch to specify output directory (so I could test my changes
more easily).

Somewhere around here I've got the start of regression suite for h2ph,
remind me to dust it off for _50.

Another bug in h2ph it would be nice to fix:

    $ echo "
    	#define FOO (BAR || BAZ)
	#if FOO
	#endif" | h2ph | perl
    Undefined subroutine &main::BAR called at - line 1.

I can't regenerate my vendor's current headers due to this one.

p5p-msgid: pz2088w5ot.fsf@eeyore.ibcinc.com