LLVM/Clang 11 automatically adds "-Wundef-prefix=TARGET_OS_" as default compiler
argument and there are several instances in the ChemDraw code where things are not defined.
--- External/ChemDraw/CMakeLists.txt.orig	2026-03-02 13:49:31
+++ External/ChemDraw/CMakeLists.txt	2026-03-02 13:50:28
@@ -123,7 +123,7 @@
   endif()

   if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wformat-overflow=0 -Wformat=0 -Wno-format-security")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wformat-overflow=0 -Wformat=0 -Wno-format-security -Wno-undef-prefix")
   endif()

   include_directories(chemdraw)