When "apcu_clear_cache()" exists, prefer it as a cache clear callback over "apc_clear_cache()"

Summary:
See PHI36. APCu originally had `apc_` methods, but at some point dropped these and only provides `apcu_` methods.

When the `apcu_` method is present, use it. It may not be present for older versions of APCu, so keep the fallback.

Test Plan:
  - With modern APCu, clicked "Purge Caches" in Config > Caches.
  - Before: fatal on bad `apc_clear_caches` call.
  - After: Valid cache clear.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D18449

src/applications/cache/spec/PhabricatorDataCacheSpec.php

@@ -50,9 +50,15 @@ final class PhabricatorDataCacheSpec extends PhabricatorCacheSpec {
       ->setVersion(phpversion('apcu'));
 
     if (ini_get('apc.enabled')) {
+      if (function_exists('apcu_clear_cache')) {
+        $clear_callback = 'apcu_clear_cache';
+      } else {
+        $clear_callback = 'apc_clear_cache';
+      }
+
       $this
         ->setIsEnabled(true)
-        ->setClearCacheCallback('apc_clear_cache');
+        ->setClearCacheCallback($clear_callback);
       $this->initAPCCommonSpec();
     } else {
       $this->setIsEnabled(false);