[boost] lots of warning with gcc-4.8.1 + boost-1.54.0

[Nathan Ridge]

> Lots of warnings like this:
>
> ./boost/bind/arg.hpp:37:22: warning: typedef ‘T_must_be_placeholder’ locally
> defined but not used [-Wunused-local-typedefs]
>
> when building 1.54.0 with gcc-4.8.1 (fedora f19)

This warning is new in GCC 4.8, and I have been seeing a ton of them
not just in boost, but in every codebase I've built.

IMO gcc should not have this warning turned on by default.

CC'ing the gcc folks.

Regards,
Nate