This worked for older qttools with libcxx23 because el_font.cpp used to
include html.h rather than document.h, which then transitively included
stdlib.
--- a/src/assistant/qlitehtml/src/3rdparty/litehtml/src/el_font.cpp
+++ b/src/assistant/qlitehtml/src/3rdparty/litehtml/src/el_font.cpp
@@ -1,2 +1,3 @@
+#include <cstdlib>
 #include "el_font.h"
 #include "document.h"