Do not use __ptr_t

[Paul Eggert]

Zack Weinberg wrote:
> Macros can be deprecated using _Pragma("GCC message \"...\"").

Thanks, good to know. So, something like this instead:

/* This is not a typedef so 'const __ptr_t' does the right thing.  */
#define __ptr_t void * __glibc_macro_warning ("__ptr_t is deprecated")