Boost 1.72: Powerpc compiler gives error on including unordered_map

When including unordered_map.hpp file, the powerpc compiler (gcc 4.7.2) gives an error inside the tuple_basic.hpp.
I am actually upgrading boost on the project from 1.42 to 1.72. I dont see any errors from the 1.42 version.

```
In file included from /boost/1_72_0/include/boost/tuple/tuple.hpp:28:0,
from /boost/1_72_0/include/boost/unordered/detail/implementation.hpp:31,
from /boost/1_72_0/include/boost/unordered/detail/map.hpp:6,
from /boost/1_72_0/include/boost/unordered/unordered_map.hpp:21,
from /boost/1_72_0/include/boost/unordered_map.hpp:17,
from build/ppc2/system/util/WeakReference.h:17,
from build/ppc2/system/util/WeakReference.c++:8:
/boost/1_72_0/include/boost/tuple/detail/tuple_basic.hpp:704:60: error: ignoring attributes on template argument 'void (boost::tuples::detail::swallow_assign::*(void (boost::tuples::detail::swallow_assign::*)()))()' [-Werror]
/boost/1_72_0/include/boost/tuple/detail/tuple_basic.hpp:836:37: error: ignoring attributes on template argument 'void (boost::tuples::detail::swallow_assign::*(void (boost::tuples::detail::swallow_assign::*)()))()' [-Werror]
```