APC cache_prefix during bootstrap

When using one apc instance for multiple installations (not multisite instances) the apc cache prefix can still cause problems:
The prefix is used from the variables and they seem not to be available during bootstrap.
Whenever two different installations save their bootstrap functions into apc this causes problems.

As a solution I would propose a simple prefix when the variable cache_prefix is not yet available.
The patch attached uses the installation directory as fallback.