G++ 5 does not implement the _Atomic keyword, which is part of C11 but not
C++11, so the existing <stdatomic.h> based atomic implementation doesn't
work.  This commit adds a new implementation based on the C++11 <atomic>
header.

In this area, C++ is pickier about types than C, so a few of the
definitions in ovs-atomic.h have to be updated to use more precise types
for integer constants.

This updates the code that generates cxxtest.cc to #include <config.h>
(so that HAVE_ATOMIC is defined) and to automatically regenerate when the
program is reconfigured (because otherwise the #include <config.h>) won't
get added without a "make clean" step).

"ovs-atomic.h" is not a public header, but apparently some code was
using it anyway.

Fixes: 9c463631

 ("ovs-atomic: Report error for contradictory configuration.")
Reported-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>