diff --git a/src/tools/tools.pro b/src/tools/tools.pro
index be4170db1ae..fe9f3a73970 100644
--- a/src/tools/tools.pro
+++ b/src/tools/tools.pro
@@ -23,7 +23,7 @@ exists($$LLVM_INSTALL_DIR) {
     SUBDIRS += clangbackend
 }
 
-BUILD_CPLUSPLUS_TOOLS = $$(BUILD_CPLUSPLUS_TOOLS)
+isEmpty(BUILD_CPLUSPLUS_TOOLS):BUILD_CPLUSPLUS_TOOLS=$$(BUILD_CPLUSPLUS_TOOLS)
 !isEmpty(BUILD_CPLUSPLUS_TOOLS) {
     SUBDIRS += cplusplus-ast2png \
         cplusplus-frontend \