build: optionally build and install with meson

This commit adds several meson.build, which successfully build and
install Lix executables, libraries, and headers. Meson does not yet
build docs, Perl bindings, or run tests, which will be added in
following commits. As such, this commit does not remove the existing
build system, or make it the default, and also as such, this commit has
several FIXMEs and TODOs as notes for what should be done before the
existing autoconf + make buildsystem can be removed and Meson made the
default. This commit does not modify any source files.

A Meson-enabled build is also added as a Hydra job, and to
`nix flake check`.

Change-Id: I667c8685b13b7bab91e281053f807a11616ae3d4

src/nix/meson.build

@@ -0,0 +1,119 @@
+generate_manpage_gen = gen_header.process(meson.project_source_root() / 'doc/manual/generate-manpage.nix')
+
+utils_gen = gen_header.process(meson.project_source_root() / 'doc/manual/utils.nix')
+
+get_env_gen = gen_header.process('get-env.sh')
+
+# src/nix/profile.cc includes src/nix/profile.md, which includes "doc/files/profiles.md.gen.hh".
+# Unfortunately, https://github.com/mesonbuild/meson/issues/2320.
+# "docs/files" isn't a directory hierarchy that already exists somewhere in this source tree,
+# and Meson refuses to create targets with specific directory paths.
+# So run_command() it is.
+# NOTE(Qyriad): This corresponds to the previous buildsystem's `src/nix/doc/files/%.md` rule,
+# which as far as I can tell was only used for this file.
+run_command(
+  installcmd,
+  '-D',
+  meson.project_source_root() / 'doc/manual/src/command-ref/files/profiles.md',
+  meson.current_build_dir() / 'doc/files/profiles.md',
+  check : true,
+)
+profiles_md_gen = gen_header.process(
+  meson.current_build_dir() / 'doc/files/profiles.md',
+  preserve_path_from : meson.current_build_dir(),
+)
+
+nix_sources = files(
+  'add-to-store.cc',
+  'app.cc',
+  'build.cc',
+  'bundle.cc',
+  'cat.cc',
+  'copy.cc',
+  'daemon.cc',
+  'derivation-add.cc',
+  'derivation-show.cc',
+  'derivation.cc',
+  'develop.cc',
+  'diff-closures.cc',
+  'doctor.cc',
+  'dump-path.cc',
+  'edit.cc',
+  'eval.cc',
+  'flake.cc',
+  'fmt.cc',
+  'hash.cc',
+  'log.cc',
+  'ls.cc',
+  'main.cc',
+  'make-content-addressed.cc',
+  'nar.cc',
+  'optimise-store.cc',
+  'path-from-hash-part.cc',
+  'path-info.cc',
+  'ping-store.cc',
+  'prefetch.cc',
+  'profile.cc',
+  'realisation.cc',
+  'registry.cc',
+  'repl.cc',
+  'run.cc',
+  'search.cc',
+  'show-config.cc',
+  'sigs.cc',
+  'store-copy-log.cc',
+  'store-delete.cc',
+  'store-gc.cc',
+  'store-repair.cc',
+  'store.cc',
+  'upgrade-nix.cc',
+  'verify.cc',
+  'why-depends.cc',
+)
+
+nix = executable(
+  'nix',
+  nix_sources,
+  generate_manpage_gen,
+  utils_gen,
+  get_env_gen,
+  profiles_md_gen,
+  nix2_commands_sources,
+  dependencies : [
+    liblixcmd,
+    liblixutil,
+    liblixstore,
+    liblixexpr,
+    liblixfetchers,
+    liblixmain,
+    boehm,
+  ],
+  install : true,
+  # FIXME(Qyriad): is this right?
+  install_rpath : libdir,
+)
+
+nix_symlinks = [
+  'nix-build',
+  'nix-channel',
+  'nix-collect-garbage',
+  'nix-copy-closure',
+  'nix-daemon',
+  'nix-env',
+  'nix-hash',
+  'nix-instantiate',
+  'nix-prefetch-url',
+  'nix-shell',
+  'nix-store',
+]
+
+foreach linkname : nix_symlinks
+  install_symlink(
+    linkname,
+    # TODO(Qyriad): should these continue to be relative symlinks?
+    pointing_to : 'nix',
+    install_dir : bindir,
+    # The 'runtime' tag is what executables default to, which we want to emulate here.
+    install_tag : 'runtime'
+  )
+endforeach