* (bug 30131) XCache with variable caching disabled no longer used for variable cachi...

* (bug 30131) XCache with variable caching disabled no longer used for variable caching (CACHE_ACCEL)

Patch from John Du Hart: https://bugzilla.wikimedia.org/attachment.cgi?id=8849&action=diff

If XCache is present, but the xcache.var_size setting is off, we'll now consider it as xcache being disabled for CACHE_ACCEL purposes. It won't trigger view of the accelerator option in the installer, and at runtime if using CACHE_ACCEL and no other modules are available, it'll throw an error so you know that it doesn't work!