Add -custom-attribute

src/build_settings.cpp

@@ -862,6 +862,8 @@ struct BuildContext {
 	bool   keep_object_files;
 	bool   disallow_do;
 
+	StringSet custom_attributes;
+
 	bool   strict_style;
 
 	bool   ignore_warnings;
@@ -1882,6 +1884,8 @@ gb_internal void init_build_context(TargetMetrics *cross_target, Subtarget subta
 		bc->thread_count = gb_max(bc->affinity.thread_count, 1);
 	}
 
+	string_set_init(&bc->custom_attributes);
+
 	bc->ODIN_VENDOR  = str_lit("odin");
 	bc->ODIN_VERSION = ODIN_VERSION;
 	bc->ODIN_ROOT    = odin_root_dir();