test Darwin 8 atomic(3)

auto/os/conf

@@ -42,6 +42,16 @@ case "$NGX_PLATFORM" in
         CORE_INCS="$UNIX_INCS"
         CORE_DEPS="$UNIX_DEPS $POSIX_DEPS"
         CORE_SRCS="$UNIX_SRCS"
+
+        ngx_feature="atomic(3)"
+        ngx_feature_name=NGX_DARWIN_ATOMIC
+        ngx_feature_run=no
+        ngx_feature_incs="#include <libkern/OSAtomic.h>"
+        ngx_feature_path=
+        ngx_feature_libs=
+        ngx_feature_test="int32_t  lock, n;
+                          n = OSAtomicCompareAndSwap32Barrier(0, 1, lock)"
+        . auto/feature
     ;;
 
     HP-UX:*)