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This tweaks the recently-added GnuTLS improvements so that
they avoid some integer-overflow problems and follow typical
Emacs style a bit better.
* configure.ac (HAVE_GNUTLS3_HMAC, HAVE_GNUTLS3_AEAD)
(HAVE_GNUTLS3_CIPHER): Use AC_CACHE_CHECK so that the
configure-time results are displayed.
* src/fns.c (extract_data_from_object): Return char *, not char
const *, since one gnutls caller wants a non-const pointer.  Use
CONSP rather than !NILP when testing for conses.  Use CAR_SAFE
instead of rolling our own code.  Prefer signed types to unsigned
when either will do.  Report problems for lengths out of range,
instead of silently mishandling them.
* src/gnutls.c (emacs_gnutls_strerror): New function, to simplify
callers.  All callers of gnutls_sterror changed.
(Fgnutls_boot): Check for integers out of range rather than
silently truncating them.
(gnutls_symmetric_aead): Check for integer overflow in size
calculations.
(gnutls_symmetric_aead, Fgnutls_macs, Fgnutls_digests):
Prefer signed to unsigned integers where either	will do.
(gnutls_symmetric_aead, gnutls_symmetric):
Work even if ptrdiff_t is wider than ‘long’.
(gnutls_symmetric, Fgnutls_hash_mac, Fgnutls_hash_digest):
Check for integer overflow in algorithm selection.