diff --git a/dtool/metalibs/dtoolconfig/CMakeLists.txt b/dtool/metalibs/dtoolconfig/CMakeLists.txt
index 22ed64ed6d..a9ed486379 100644
--- a/dtool/metalibs/dtoolconfig/CMakeLists.txt
+++ b/dtool/metalibs/dtoolconfig/CMakeLists.txt
@@ -1,3 +1,5 @@
+add_definitions(-DBUILDING_DTOOLCONFIG)
+
 if(HAVE_PYTHON)
   add_library(p3dtoolconfig dtoolconfig.cxx pydtool.cxx)
   target_link_libraries(p3dtoolconfig ${PYTHON_LIBRARIES})