Fix some -Wundef issues.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@228266 91177308-0d34-0410-b5e6-96231b3b80d8
Change-Id: Ib53cc3c6d89254376a34d795f6bf34b32f7c1577
(cherry picked from commit 3727be521a9b4c5fcf95c53adc8abe85f7105188)
diff --git a/include/__refstring b/include/__refstring
index 6866bf1..61ccc75 100644
--- a/include/__refstring
+++ b/include/__refstring
@@ -13,7 +13,7 @@
 #include <__config>
 #include <cstddef>
 #include <cstring>
-#if __APPLE__
+#ifdef __APPLE__
 #include <dlfcn.h>
 #include <mach-o/dyld.h>
 #endif
@@ -49,7 +49,7 @@
         return data + sizeof(*rep);
     }
 
-#if __APPLE__
+#ifdef __APPLE__
     static
     const char*
     compute_gcc_empty_string_storage() _NOEXCEPT