Disable deprecated-declarations warning on Clang.

The Luabind headers currently make use of std::auto_ptr, which generates
tons of warnings in Clang 3.4.  Our previous attempts to replace
auto_ptr with unique_ptr (via a somewhat messy merge from Luaponte)
silenced the warnings, but causes runtime errors with class
registration.

We'll live with the specific warnings disabled for now until we make a
decision about whether we should switch to a different Luabind fork.