Relative #include directories are resolve to be relative to the directory in which the build executes

src/dds/build/plan/compile_file.cpp

 compile_command_info compile_file_plan::generate_compile_command(build_env_ref env) const {
     compile_file_spec spec{_source.path, calc_object_file_path(env)};
     spec.enable_warnings = _rules.enable_warnings();
-    extend(spec.include_dirs, _rules.include_dirs());
+    for (auto dirpath : _rules.include_dirs()) {
+        if (!dirpath.is_absolute()) {
+            dirpath = env.output_root / dirpath;
+        }
+        dirpath = fs::weakly_canonical(dirpath);
+        spec.include_dirs.push_back(std::move(dirpath));
+    }
     for (const auto& use : _rules.uses()) {
         extend(spec.external_include_dirs, env.ureqs.include_paths(use));
     }