Define config var PHP_JSON

Otherwise, `ADD_EXTENSION_DEP('foo', 'json')` fails, even though the
JSON extension is available.

ext/json/config.w32

@@ -1,7 +1,7 @@
 // vim:ft=javascript
 
 EXTENSION('json', 'json.c', false /* never shared */, "/DZEND_ENABLE_STATIC_TSRMLS_CACHE=1");
-
+PHP_JSON="yes";
 ADD_SOURCES(configure_module_dirname, "json_encoder.c json_parser.tab.c json_scanner.c", "json");
 
 ADD_MAKEFILE_FRAGMENT();