Compilation on MacOS fixes, OpenSSL upgrade and warning fixes.

[romanbsd]

No description provided.

[romanbsd]

@troglobit what do you think?

[troglobit]

I think there's a lot going on here, before I do a proper line-by-line review, this single commit needs to be:

1. split up in logical changes, with relevant commit messages detailing why changes are made, at least split macOS fixes from OpenSSL upgrade, as well as docker, build.sh and similar in separate commits
2. specific API replacements should go into lib/, e.g., mkostemp() and getdtablesize(), instead of adding a macOS compatible change inline

Don't get me wrong, even though Merecat is a low-prio project for me, I still use it. So I really appreciate your taking time to upgrade to OpenSSL 3.x. For my own reasons I need to support Ubuntu 22.04, meaning autconf 2.71 and OpenSSL 3.0.2, meaning a very slight adjustment is needed in configure.ac

[romanbsd]

The changes are supposed to be cross-platform compatible, using standard apis. Having said that, if there's some interest, I can split to different commits. Does it compile as is on your Linux?

[troglobit]

Well, one can always debate the finer points of cross-platform support, but I've made it very easy to add replacements for GNU APIs, so I'd like for contributors to honor that.

Yes there's interest for these changes, and it builds on Ubuntu 24.04, with the exception of the configure.ac bump of autoconf -> 2.72.