Re: ecpg: issue related to preprocessor directives

I wrote:
> Looking at pgc.l, it seems that 'elif' is treated as though it were
> 'endif' followed by 'ifdef', which of course completely loses the
> expected property that a previous successful branch would keep the
> elif branch from being expanded.
> While this doesn't look terribly hard to fix, I'm a little disturbed
> by the fact that the existing semantics seem to date back to 1999
> (b57b0e044). We're probably risking breaking existing app code if
> we change it. I think we *should* change it, of course, but I'm kind
> of inclined not to back-patch.

Here's a proposed patch, which also clarifies the documentation,
which seemed a bit confused/misleading to me.

As stated, I'm not sure it's wise to back-patch this aggressively
... but maybe it'd be okay to squeeze it into v13?

regards, tom lane