Using `std::type_info::operator==` fails under libc++ because the .so
is loaded with RTLD_LOCAL. libc++ considers types under such .sos
distinct, and so comparing typeid() values directly isn't going to work.
This adds a custom hasher and equality class for the type lookup maps
when not under stdlibc++, and adds a `detail::same_type` function to
perform the equality test. It also converts a few pointer arguments to
const lvalue references, particularly since doing the pointer
comparison wasn't technically valid to being with (though in practice,
appeared to work everywhere).
This fixes#912.
Jason Rhinelander
2196696746