[CMake] Doxygen setup, fixed Python var.

scripts/cmake/DocumentationSetup.cmake

@@ -49,7 +49,7 @@ doxygen_add_docs(doc
     ${PROJECT_SOURCE_DIR}/
     ${PROJECT_BINARY_DIR}/DocAux/dox)
 
-if (BASH_TOOL_PATH AND PYTHON_EXECUTABLE)
+if (BASH_TOOL_PATH AND Python_EXECUTABLE)
     set(doc_use_external_tools TRUE)
 else()
     set(doc_use_external_tools FALSE)