Respect --enable-warnings

src/dds/build.cpp

         = load_usage_requirements(params.root, params.out_root, params.lm_index);
 
     library_build_params lib_params;
-    lib_params.build_tests = params.build_tests;
-    lib_params.build_apps  = params.build_apps;
+    lib_params.build_tests     = params.build_tests;
+    lib_params.build_apps      = params.build_apps;
+    lib_params.enable_warnings = params.enable_warnings;
     for (const library& lib : libs) {
         lib_params.out_subdir = fs::relative(lib.path(), params.root);
         pkg.add_library(library_plan::create(lib, lib_params, ureqs));

src/dds/build/plan/library.cpp

     }
 
     auto compile_rules = lib.base_compile_rules();
+    compile_rules.enable_warnings() = params.enable_warnings;
     for (const auto& use : lib.manifest().uses) {
         ureqs.apply(compile_rules, use.namespace_, use.name);
     }

src/dds/library.hpp

 
 struct library_build_params {
     fs::path out_subdir;
-    bool     build_tests = false;
-    bool     build_apps  = false;
+    bool     build_tests     = false;
+    bool     build_apps      = false;
+    bool     enable_warnings = false;
 };
 
 std::vector<library> collect_libraries(path_ref where);