libctf: make it compile for old glibc

[Nick Alcock]

On 15 Jul 2019, Hans-Peter Nilsson stated:

> On Sun, 14 Jul 2019, Nick Alcock wrote:
>
>> On 11 Jul 2019, Hans-Peter Nilsson spake thusly:
>> > But objdump should be able to decode CTF generated by outside
>> > producers, right?
>>
>> Yes, but right now there are only two: libctf and GCC. So it's hard to
>> test it against a third producer, when no third exists :) though I'd
>> agree that when a third *does* exist, we should generate some CTF with
>> it and make sure that at the very least we can read it in with libctf.
>
> The posted links made me think CTF was in heavy use somewhere in
> BSD-land and for some Solaris thingy since years already (dtrace)?
>
> Is it, but for an incompatible version?  If so, supporting the
> pre-existing versions (at least reading them) will probably be
> requested sooner rather than later.

It is, but for an incompatible version -- and adding support for reading
that version, and indeed writing all versions that it is possible to
write without losing information, is on my todo list and isn't even all
that hard one I've done a few internal refactorings I have to do anyway.

(Their versions have an incompatible versioning scheme, but consistently
store their data in a differently-named section, so should always be
unambiguously distinguishable from ours.)