meson: make it possible to compile without dbus

Make an option to disable dbus and all the code that depends on
it.

Fixes #1685

test/meson.build

@@ -74,8 +74,6 @@ test('test context',
                'test-config.c',
                include_directories: pwtest_inc,
                link_with: [pwtest_lib,
-                            spa_support_lib,
-                            spa_dbus_lib,
                             pipewire_module_protocol_native,
                             pipewire_module_client_node,
                             pipewire_module_client_device,

test/test-context.c

@@ -227,7 +227,9 @@ PWTEST(context_support)
 		SPA_TYPE_INTERFACE_Loop,
 		SPA_TYPE_INTERFACE_LoopUtils,
 		SPA_TYPE_INTERFACE_Log,
+#if HAVE_DBUS
 		SPA_TYPE_INTERFACE_DBus,
+#endif
 		SPA_TYPE_INTERFACE_CPU
 	};