Add Clang compiler support and update project to C17 standard

[DL6ER]

What does this implement/fix?

Add Clang compiler support, tested with

• Clang 14.0.0 (Ubuntu 22.04.3 LTS),
• Clang 16.0.2 (Alpine 3.18), and
• Clang 17.0.6 (Alpine Edge)

C17 is the informal name for ISO/IEC 9899:2018 and was published in June 2018. It replaced C11 and will eventually be superseded by C23 when it is published. C17 fixes numerous minor defects in C11 without introducing new language features. It required a number of subtle code changes without any real changes. We also bump the required minimum CMake version to the lowest version supporting the C17 standard.

---

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.

[DL6ER]

Opening this PR in draft mode as I first want to add another CI step that ensures compilation fails on clang warnings as well. FOr this, we need to create new ftl-build containers including clang.

Unfortunately, there is currently a bug in alpine:edge that prevents us from creating new containers. While they updated gcc, they did not recompile libgmp using this new compiler, so far. This leads to a fatal error during linking time:

lto1: fatal error: bytecode stream in file '/usr/lib/gcc/x86_64-alpine-linux-musl/13.2.1/../../../../lib/libgmp.a' generated with LTO version 13.0 instead of the expected 13.1
compilation terminated.
lto-wrapper: fatal error: /usr/bin/cc returned 1 exit status
compilation terminated.
/usr/lib/gcc/x86_64-alpine-linux-musl/13.2.1/../../../../x86_64-alpine-linux-musl/bin/ld: error: lto-wrapper failed
collect2: error: ld returned 1 exit status

The proper fix for this to have the alpine team recompile libgmp as they seem to have done with most other packages already. See similar error 1 and 2 on ArchLinux.

[DL6ER]

Bug report opened upstream https://gitlab.alpinelinux.org/alpine/aports/-/issues/16121

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions]

Conflicts have been resolved.

[DL6ER]

The upstream bug has been fixed in alpine:edge via alpine/aports@6867589a.

[DL6ER]

This PR is done. It now awaits the decision on pi-hole/docker-base-images#94 and will then uses the tagged container (v2.6) as CI build environment.

[DL6ER]

Ready for review + merge

[yubiuser]

There are a few places where you removed some counter variables - why? Were they not used anymore and you just noticed yet?

E.g. totalcount in stats.c, num_clients in stats_database.c and elements ind validator.c

[DL6ER]

All changes were some due to compiler warnings triggered by the new C standard and additional warnings due to using clang next to gcc. No functional changes have been made.

I'm the particular case you found they were actually never used and clang complained about work being some without getting the result of it at any point. Assembly inspection showed that gcc chose to instead optimize these computations away silently.