[Bug c++/60976] Compilation with G++ 4.9.0 is 2-3 times slower than with 4.8.2

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60976

--- Comment #25 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There was a G++ bug (now fixed) that made void_t not work, try this alternative
version:

template< class... > struct __voider { using type = void; };
template< class... _T0toN > using __void_t = typename
__voider<_T0toN...>::type;