diff --git a/include/__config_site.in b/include/__config_site.in
index 7c7c226..580a6aa 100644
--- a/include/__config_site.in
+++ b/include/__config_site.in
@@ -28,6 +28,7 @@
 #cmakedefine _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL
 #cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
 #cmakedefine _LIBCPP_NO_VCRUNTIME
+#cmakedefine _LIBCPP_ABI_NAMESPACE @_LIBCPP_ABI_NAMESPACE@
 
 @_LIBCPP_ABI_DEFINES@