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)

src/hydra-evaluator/meson.build

@@ -0,0 +1,9 @@
+hydra_evaluator = executable('hydra-evaluator',
+  'hydra-evaluator.cc',
+  dependencies: [
+    libhydra_dep,
+    nix_dep,
+    pqxx_dep,
+  ],
+  install: true,
+)