dnsdist: Add meson support

[rgacogne]

Short description

This PR is unfortunately duplicating quite a lot of sub-directories from the root meson directory used by the auth and rec, because we are either using them in a different way or the description is not accurate for dnsdist. It might make sense to use symlinks for the directories we are not modifying?

Closes #14654

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 11405281222

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 298 of 462 (64.5%) changed or added relevant lines in 24 files are covered.
• 9901 unchanged lines in 132 files lost coverage.
• Overall coverage decreased (-2.0%) to 62.668%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/dnsdistdist/bpf-filter.cc	0	1	0.0%
pdns/dnsdistdist/dnsdist-tcp-upstream.hh	0	1	0.0%
pdns/dnsdistdist/test-dnsdist-lua-ffi.cc	0	1	0.0%
pdns/dnsdistdist/dnsdist-kvs.hh	0	2	0.0%
pdns/dnsdistdist/dnsdist-nghttp2-in.cc	15	17	88.24%
pdns/dnsdistdist/dnsdist-lbpolicies.cc	0	3	0.0%
pdns/dnsdistdist/dnsdist-lua-ffi.cc	0	3	0.0%
pdns/dnsdistdist/dnsdist-nghttp2.cc	11	14	78.57%
pdns/dnsdistdist/test-dnsdistnghttp2_common.hh	9	12	75.0%
pdns/tcpiohandler.cc	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
pdns/webserver.hh	1	66.33%
pdns/dnsdistdist/test-dnsdistlbpolicies_cc.cc	1	79.09%
pdns/dnsdistdist/test-dnsdistnghttp2_common.hh	2	73.45%
pdns/ws-api.cc	2	84.91%
ext/yahttp/yahttp/utility.hpp	2	36.53%
pdns/burtle.hh	2	97.55%
pdns/lock.hh	2	84.52%
pdns/dnsdistdist/dnsdist-lua-bindings-dnsquestion.cc	2	0.0%
pdns/channel.hh	2	45.89%
pdns/dnsdistdist/test-dnsdistnghttp2-in_cc.cc	2	83.42%

Totals
Change from base Build 11402945382:	-2.0%
Covered Lines:	104352
Relevant Lines:	144412

---

💛 - Coveralls

[omoerbeek]

I tested macOS (dnsdist + testrunner, no further options), no changes needed.

This patch makes basic build of dnsdist + testrunner succeed on OpenBSD:
x.txt

[rgacogne]

Applied, thanks!