build runner: implement --watch (work-in-progress)

I'm still learning how the fanotify API works but I think after playing
with it in this commit, I finally know how to implement it, at least on
Linux. This commit does not accomplish the goal but I want to take the
code in a different direction and still be able to reference this point
in time by viewing a source control diff.

I think the move is going to be saving the file_handle for the parent
directory, which combined with the dirent names is how we can correlate
the events back to the Step instances that have registered file system
inputs. I predict this to be similar to implementations on other
operating systems.

lib/std/Build/Step/InstallFile.zig

@@ -39,7 +39,10 @@ fn make(step: *Step, prog_node: std.Progress.Node) !void {
     _ = prog_node;
     const b = step.owner;
     const install_file: *InstallFile = @fieldParentPtr("step", step);
-    step.addWatchInput(install_file.source);
+
+    // Inputs never change when re-running `make`.
+    if (!step.inputs.populated()) step.addWatchInput(install_file.source);
+
     const full_src_path = install_file.source.getPath2(b, step);
     const full_dest_path = b.getInstallPath(install_file.dir, install_file.dest_rel_path);
     const cwd = std.fs.cwd();