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)

doc/manual/Makefile.am

@@ -1,6 +0,0 @@
-MD_FILES = src/*.md
-
-EXTRA_DIST = $(MD_FILES)
-
-install: $(MD_FILES)
-	mdbook build . -d $(docdir)

doc/manual/meson.build

@@ -0,0 +1,36 @@
+srcs = files(
+  'src/SUMMARY.md',
+  'src/about.md',
+  'src/api.md',
+  'src/configuration.md',
+  'src/hacking.md',
+  'src/installation.md',
+  'src/introduction.md',
+  'src/jobs.md',
+  'src/monitoring/README.md',
+  'src/notifications.md',
+  'src/plugins/README.md',
+  'src/plugins/RunCommand.md',
+  'src/plugins/declarative-projects.md',
+  'src/projects.md',
+  'src/webhooks.md',
+)
+
+manual = custom_target(
+  'manual',
+  command: [
+    mdbook,
+    'build',
+    '@SOURCE_ROOT@/doc/manual',
+    '-d', meson.current_build_dir() / 'html'
+  ],
+  depend_files: srcs,
+  output: ['html'],
+  build_by_default: true,
+)
+
+install_subdir(
+  manual.full_path(),
+  install_dir: get_option('datadir') / 'doc/hydra',
+  strip_directory: true,
+)