Enable -Wundef.

The problem that caused the need for http://reviews.llvm.org/D7419 was
caused by testing the value of something that was undefined. This
should prevent that in the future.


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@228257 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/cxxabi.h b/include/cxxabi.h
index 2614327..ae75f86 100644
--- a/include/cxxabi.h
+++ b/include/cxxabi.h
@@ -77,7 +77,7 @@
 extern LIBCXXABI_NORETURN void __cxa_deleted_virtual(void);
 
 // 3.3.2 One-time Construction API
-#if __arm__
+#ifdef __arm__
 extern int  __cxa_guard_acquire(uint32_t*);
 extern void __cxa_guard_release(uint32_t*);
 extern void __cxa_guard_abort(uint32_t*);