* libc/ctype/ctype_c.c: Move inclusion of ctype_iso.h and
ctype_cp.h out of ALLOW_NEGATIVE_CTYPE_INDEX case.
(__ctype_ptr__): Constify in !_MB_CAPABLE case. Otherwise,
de-constify in !ALLOW_NEGATIVE_CTYPE_INDEX case, too. Add comment.
(__set_ctype): Set __ctype_ptr__ pointer according to definition
of ALLOW_NEGATIVE_CTYPE_INDEX.
* libc/include/ctype.h (__ctype_ptr__): Constify in !_MB_CAPABLE case.