Add Full APC compatibility mode

See https://github.com/krakjoe/apcu/issues/7

This is a first step.
When build with --enable-apc-bc

    $ php -m 
    [PHP Modules]
    apc
    apcu
    Core
    ...

    $ php  -r 'print_r(get_loaded_extensions());'
    Array
    (
        [0] => Core
        ...
        [15] => apcu
        [16] => mhash
        [17] => apc
    )