allow to build with APC or APCU

With this quite simple patch, igbinary will detect at buildtime if APCU is installed and will use it.
Else, keep the previous behavior (APC).

Notice : for now, when used, ACPU is REQUIRED. 
I hope we could improves this, switching to previous APC behavior.

See https://github.com/krakjoe/apcu/issues/24