Fixes compilation with clang

Though, one also needs to modify the non-boost asio a bit to
support std::errors. In particular, one needs to make sure that
the define ASIO_HAS_CLANG_LIBCXX is set to true (1) in
MaidSafe/src/third_party_libs/header_only/asio/detail/config.hpp
Line 79

This currently isn't the case by default, I set it to true manually
and will have a look at why it's not set later.