libctf: make it compile for old glibc

[Nick Alcock]

On 14 Jul 2019, Nick Alcock verbalised:

> On 11 Jul 2019, Hans-Peter Nilsson spake thusly:
>
>> On Thu, 11 Jul 2019, Nick Alcock wrote:
>>> On 11 Jul 2019, Hans-Peter Nilsson stated:
>>> I have a big pile of stuff built up to be pushed soon (ctf linking work,
>>> mostly in libctf but a bit in ld): I can add your commit to it if you
>>> like,
>>
>> Yes, please.  Thanks.
>
> I'll add it into the linker batch's preparatory work. (16 commits
> already...)

Done, though it's not all really *applicable* yet because I noticed
while prepping it for sending that the linker changes were broken on
non-ELF, so I have to fix that first. (And for all I know, everyone else
will think my approach to the linker changes is utterly horrific and
must be totally rewritten :) I hope not, but it's a definite
possibility.)

However, hopefully the earlier parts of the series, including your
commit, will be pushable soonish.

Thanks for the fix!

-- 
NULL && (void)