Fix malloc_hook warning on glibc 2.17 and later.

In glib 2.17 the __malloc_ptr define was removed in favour
of using void* directly. Our declaration of mhook using this
type for the second argument therefore rightly generates a warning
on modern systems, since the type is assumed to be an int, which
is too narrow to hold a pointer on 64 bit architectures.

Since it was only ever a define we could use an #ifndef to define
__malloc_ptr ourselves. However we only use it once, so using void*
in the signature directly is cleaner. This should cause no problems
on older systems where it will match either the void* or the char*
(for non __STDC__ code) the define resolved to.