autotools -> meson

Original commit message:

> There are some known regressions regarding local testing setups - since
> everything was kinda half written with the expectation that build dir =
> source dir (which should not be true anymore). But everything builds and
> the test suite runs fine, after several hours spent debugging random
> crashes in libpqxx with MALLOC_PERTURB_...

I have not experienced regressions with local testing.

(cherry picked from commit 4b886d9c45cd2d7fe9b0a8dbc05c7318d46f615d)

meson.build

@@ -0,0 +1,40 @@
+project('hydra', 'cpp',
+  version: files('version.txt'),
+  license: 'GPL-3.0',
+  default_options: [
+    'debug=true',
+    'optimization=2',
+    'cpp_std=c++20',
+  ],
+)
+
+nix_store_dep = dependency('nix-store', required: true)
+nix_main_dep = dependency('nix-main', required: true)
+nix_expr_dep = dependency('nix-expr', required: true)
+nix_flake_dep = dependency('nix-flake', required: true)
+nix_cmd_dep = dependency('nix-cmd', required: true)
+
+# Nix need extra flags not provided in its pkg-config files.
+nix_dep = declare_dependency(
+  dependencies: [
+    nix_store_dep,
+    nix_main_dep,
+    nix_expr_dep,
+    nix_flake_dep,
+    nix_cmd_dep,
+  ],
+  compile_args: ['-include', 'nix/config.h'],
+)
+
+pqxx_dep = dependency('libpqxx', required: true)
+
+prom_cpp_core_dep = dependency('prometheus-cpp-core', required: true)
+prom_cpp_pull_dep = dependency('prometheus-cpp-pull', required: true)
+
+mdbook = find_program('mdbook', native: true)
+perl = find_program('perl', native: true)
+
+subdir('doc/manual')
+subdir('nixos-modules')
+subdir('src')
+subdir('t')