[GH-238] fix to solve fatal error: apc_exist() does not exist

This issue is created automatically through a Github pull request on behalf of uran1980:

Url: https://github.com/doctrine/common/pull/238

Message:

Hi, folks. I'm running *Zend Server CE 5.6.0 5.6.0 SP4* with *PHP Version 5.3.14* and APC builtin:

Now when I run in my project based on Symfony 2.1.6 comand:
```
console doctrine:fixtures:load --verbose
```
I am stack with fatal error:
```
Fatal error: Call to undefined function Doctrine\Common\Cache\apc_exists()
```


*Possible solution are in my pull request.*

As I descovered I am not alone with this problem: http://stackoverflow.com/questions/9125904/apc-exist-does-not-exist(http://stackoverflow.com/questions/9125904/apc-exist-does-not-exist)