Prepare for stricter checking in GCC 14

[jamesjer]

GCC 14 will be stricter about such things as implicit function return types, implicit conversions between pointers and integers, and mismatched pointer types. Some warnings will become errors. The Fedora project is preparing for these changes:

• https://fedoraproject.org/wiki/Changes/PortingToModernC
• https://fedoraproject.org/wiki/Toolchain/PortingToModernC

The lwt package fails to build with the test compiler mentioned in those links. The main symptom is missing features due to a gcc run that ends with an error, even though the feature is present. The cause is mostly implicit conversions between pointers and integers. In addition, due to header streamlining, errno.h is no longer implicitly included in unix_getprotoby_getservby_job.c, so include it explicitly.

I await the results of the CI run with fear and trepidation.

[raphael-proust]

Thanks for the contribution and for the detailed explanations. I don't have the bandwidth to check this in depth rn. I'll try to get to it at some point.

[raphael-proust]

Ok I had a better look. I didn't notice at first that the bulk of the changes are on the discovery script. The changes seem ok to me. I'll merge unless someone has an objection soon.

[smorimoto]

@MisterDA Do you have any ideas?

[MisterDA]

Can't look at this for the next two weeks.

[smorimoto]

Totally fine to me!