obstack: Work around ICE with Oracle cc 12.6 (regr. 2023-12-01). * lib/obstack.in.h (__PTR_ALIGN): Use SMALL_PTRDIFF_T, instead of testing sizeof (ptrdiff_t) < sizeof (void *) at compile time. * m4/obstack.m4 (gl_FUNC_OBSTACK): Set SMALL_PTRDIFF_T. * modules/obstack (Makefile.am): Substitute SMALL_PTRDIFF_T.