Bug 876156: Don't try using <atomic> on OpenBSD which uses an old libstdc++ r=nfroyd

mfbt/Atomics.h

@@ -32,8 +32,10 @@
     * GCC.  We do not have a particularly good way to detect this sort of
     * case at this point, so just assume that if we're on a Linux system,
     * we can't use the system's <atomic>.
+    *
+    * OpenBSD uses an old libstdc++ 4.2.1 and thus doesnt have <atomic>.
     */
-#  if !defined(__linux__) && \
+#  if !defined(__linux__) && !defined(__OpenBSD__) && \
       (__cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)) && \
       __has_include(<atomic>)
 #    define MOZ_HAVE_CXX11_ATOMICS