Blocked iterator mac fixes

The two commits in this pull request allow the `test_multiarray_chunked` test to compile on Mac, but they are not sufficient to get the test to pass.

BTW, to compile on Mac, set CMAKE_CXX_FLAGS to `-std=c++11 -stdlib=libc++`

The test fails with the following output:

    Running test_multiarray_chunked
    chunked multi array test for type h: 
        contiguous array (baseline): 10.4092 msec
    chunked multi array test for type h: 
    max cache size: 200: 
    file size: 8003584
        chunked iterator create file: 35.093 msec
        chunked iterator: 20.3569 msec
    final cache size: 64
    chunked multi array test for type h: 
    max cache size: 16: 
    file size: 8003584
        chunked iterator create file: 38.3279 msec
        chunked iterator: 22.3889 msec
    final cache size: 16
    chunked multi array test for type h: 
    max cache size: 4: 
    file size: 8003584
    ./run_test_multiarray_chunked.sh: line 1: 15546 Bus error: 10           /Users/bergs/Downloads/vigra/build/test/multiarray/test_multiarray_chunked
    make[3]: *** [test/multiarray/test_multiarray_chunked] Error 1
    make[2]: *** [test/multiarray/CMakeFiles/test_multiarray_chunked.dir/all] Error 2
    make[1]: *** [test/multiarray/CMakeFiles/test_multiarray_chunked.dir/rule] Error 2
    make: *** [test_multiarray_chunked] Error 2