HPCC-12766 Add comment for ISO C++ 2011 standard

libmemcached-1.0.18 includes the header <cinttypes> which translates to
<bits/cinttypes> requiring ISO C++ 2011 standard to build.
All lesser versions explicitly include <tr1/cinttypes> which does not.

Signed-off-by: James Noss <james.noss@lexisnexis.com>
Merge pull request #6778 from jamienoss/issue12766-memcached-comment-c++11

HPCC-12766 Add comment for ISO C++ 2011 standard

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>