libstore: deprecate the build-hook setting

implementing a build hook is pretty much impossible without either being
a nix, or blindly forwarding the important bits of all build requests to
some kind of nix. we've found no uses of build-hook in the wild, and the
build-hook protocol (apart from being entirely undocumented) is not able
to convey any kind of versioning information between hook and daemon. if
we want to upgrade this infrastructure (which we do), this must not stay

Change-Id: I1ec4976a35adf8105b8ca9240b7984f8b91e147e

doc/manual/rl-next/deprecate-build-hook.md

@@ -0,0 +1,8 @@
+---
+synopsis: "The `build-hook` setting is now deprecated"
+category: Breaking Changes
+---
+
+Build hooks communicate with the daemon using a custom, internal, undocumented protocol that is entirely unversioned and cannot be changed.
+Since we intend to change it anyway we must unfortunately deprecate the current build hook infrastructure.
+We do not expect this to impact most users—we have not found any uses of `build-hook` in the wild—but if this does affect you, we'd like to hear from you!

src/libstore/globals.hh

@@ -248,14 +248,18 @@ public:
         )",
         {"build-timeout"}};
 
-    Setting<Strings> buildHook{this, {}, "build-hook",
+    Setting<Strings> buildHook{
+        AbstractSetting::deprecated_t{},
+        this, {}, "build-hook",
         R"(
           The path to the helper program that executes remote builds.
 
           Lix communicates with the build hook over `stdio` using a custom protocol to request builds that cannot be performed directly by the Nix daemon.
           The default value is the internal Lix binary that implements remote building.
 
-          > **Important**
+          > **Warning**
+          >
+          > This setting is deprecated and will be removed in a future version of Lix.
           >
           > Change this setting only if you really know what you’re doing.
         )"};