Warning when building with clang 5

[krojew]

When including scope_exit.hpp, the following warning (and thus error, if configured) is produced:

/boost/scope_exit.hpp:258:10: error: extension used [-Werror,-Wlanguage-extension-token]
#include BOOST_TYPEOF_INCREMENT_REGISTRATION_GROUP()
         ^
/boost/typeof/typeof.hpp:197:13: note: expanded from macro 'BOOST_TYPEOF_INCREMENT_REGISTRATION_GROUP'
     <boost/typeof/incr_registration_group.hpp>