Fix clang warning for unused variables (only used in ASSERT).

BUG=41
diff --git a/src/utils.h b/src/utils.h
index aa2e772..deb4bc1 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -33,7 +33,10 @@
 
 #include <assert.h>
 #ifndef ASSERT
-#define ASSERT(condition)      (assert(condition))
+#define ASSERT(condition)         \
+  do {                            \
+    assert(condition);            \
+  } while (false && (condition))
 #endif
 #ifndef UNIMPLEMENTED
 #define UNIMPLEMENTED() (abort())