Make zap() more reliable

The gcc assembly version of zap() could still be optimized out under
gcc 5.1 or later, and the krb5int_zap() function could be optimized
out with link-time optimization.  Based on work by Zhaomo Yang and
Brian Johannesmeyer, use the C11 memset_s() when available, then fall
back to a memory barrier with gcc or clang, and finally fall back to
using krb5int_zap().  Modify krb5int_zap() to use a volatile pointer
in case link-time optimization is used.

https://github.com/krb5/krb5/commit/c163275f899b201dc2807b3ff2949d5e2ee7d838
Author: Greg Hudson <ghudson@mit.edu>
Commit: c163275f899b201dc2807b3ff2949d5e2ee7d838
Branch: master
 src/aclocal.m4         |    2 ++
 src/include/k5-int.h   |   43 +++++++++++++++++++++++--------------------
 src/util/support/zap.c |    5 ++++-
 3 files changed, 29 insertions(+), 21 deletions(-)