diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -282,6 +282,10 @@
 # library.
 add_definitions(-D_LIBCXXABI_BUILDING_LIBRARY)
 
+# libcxxabi needs to, for various reasons, include the libcpp headers as if
+# it is being built as part of libcxx.
+add_definitions(-D_LIBCPP_BUILDING_LIBRARY)
+
 # Disable DLL annotations on Windows for static builds.
 if (WIN32 AND LIBCXXABI_ENABLE_STATIC AND NOT LIBCXXABI_ENABLE_SHARED)
   add_definitions(-D_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS)
diff --git a/libcxxabi/src/fallback_malloc.cpp b/libcxxabi/src/fallback_malloc.cpp
--- a/libcxxabi/src/fallback_malloc.cpp
+++ b/libcxxabi/src/fallback_malloc.cpp
@@ -6,9 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// Define _LIBCPP_BUILDING_LIBRARY to ensure _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
-// is only defined when libc aligned allocation is not available.
-#define _LIBCPP_BUILDING_LIBRARY
 #include "fallback_malloc.h"
 
 #include <__threading_support>
diff --git a/libcxxabi/src/stdlib_exception.cpp b/libcxxabi/src/stdlib_exception.cpp
--- a/libcxxabi/src/stdlib_exception.cpp
+++ b/libcxxabi/src/stdlib_exception.cpp
@@ -6,7 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-#define _LIBCPP_BUILDING_LIBRARY
 #include <new>
 #include <exception>
 
diff --git a/libcxxabi/src/stdlib_new_delete.cpp b/libcxxabi/src/stdlib_new_delete.cpp
--- a/libcxxabi/src/stdlib_new_delete.cpp
+++ b/libcxxabi/src/stdlib_new_delete.cpp
@@ -8,7 +8,6 @@
 // This file implements the new and delete operators.
 //===----------------------------------------------------------------------===//
 
-#define _LIBCPP_BUILDING_LIBRARY
 #include "__cxxabi_config.h"
 #include <new>
 #include <cstdlib>