diff --git a/contrib/jemalloc/include/jemalloc/internal/util.h b/contrib/jemalloc/include/jemalloc/internal/util.h
--- a/contrib/jemalloc/include/jemalloc/internal/util.h
+++ b/contrib/jemalloc/include/jemalloc/internal/util.h
@@ -40,6 +40,9 @@
 #  error JEMALLOC_INTERNAL_UNREACHABLE should have been defined by configure
 #endif
 
+#ifdef unreachable
+#  undef unreachable
+#endif
 #define unreachable() JEMALLOC_INTERNAL_UNREACHABLE()
 
 /* Set error code. */