BUG/MINOR: makefile: use USE_LIBATOMIC instead of USE_ATOMIC

The issue was introduced with commit c108f37c2 ("BUILD: makefile:
rework 51D to split v3/v4"), and is also related to commit b16d9b58
("BUILD: makefile: never force -latomic, set USE_LIBATOMIC instead")
where USE_ATOMIC has been replaced.
diff --git a/Makefile b/Makefile
index 1151a08..115dd55 100644
--- a/Makefile
+++ b/Makefile
@@ -690,7 +690,7 @@
     ifeq ($(USE_THREAD),)
       51DEGREES_CFLAGS += -DFIFTYONEDEGREES_NO_THREADING -DFIFTYONE_DEGREES_NO_THREADING
     endif
-    USE_ATOMIC          = implicit
+    USE_LIBATOMIC       = implicit
   endif # 51DEGREES_VER==4
 
   ifeq ($(51DEGREES_VER),3)  # v3 here