There are a few issues with the current virAtomic APIs

 - They require use of a virAtomicInt struct instead of a plain
   int type
 - Several of the methods do not implement memory barriers
 - The methods do not implement compiler re-ordering barriers
 - There is no Win32 native impl

The GLib library has a nice LGPLv2+ licensed impl of atomic
ops that works with GCC, Win32, or pthreads.h that addresses
all these problems. The main downside to their code is that
the pthreads impl uses a single global mutex, instead of
a per-variable mutex. Given that it does have a Win32 impl
though, we don't expect anyone to seriously use the pthread.h
impl, so this downside is not significant.

* .gitignore: Ignore test case
* configure.ac: Check for which atomic ops impl to use
* src/Makefile.am: Add viratomic.c
* src/nwfilter/nwfilter_dhcpsnoop.c: Switch to new atomic
  ops APIs and plain int datatype
* src/util/viratomic.h: inline impls of all atomic ops
  for GCC, Win32 and pthreads
* src/util/viratomic.c: Global pthreads mutex for atomic
  ops
* tests/viratomictest.c: Test validate to validate safety
  of atomic ops.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>