You can now say
    data {-# CTYPE "some_header.h" "the C type" #-} Foo = ...

I think it's rare that this will actually be needed. If the
header for a CAPI FFI import includes a
    void f(ctype x);
prototype then ctype must already be defined.

However, if the header only has
    #define f(p) p->j
then the type need not be defined.

But either way, it seems good practice for us to specify the header that
we need.