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.

Upstream added additional code to add "-Wno-undef-prefix" for AppleClang, but that does not
cover the situations where "macports-clang-XY" is used as compiler.

See: https://github.com/rdkit/rdkit/issues/9094
--- External/ChemDraw/CMakeLists.txt.orig	2026-05-09 21:18:08
+++ External/ChemDraw/CMakeLists.txt	2026-05-09 21:18:43
@@ -135,7 +135,7 @@
   endif()
 
   if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang" )
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-comment -Wno-parentheses -Wno-logical-op-parentheses -Wno-pointer-bool-conversion -Wno-unused-value -Wno-unsequenced -Wno-constant-logical-operand")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-comment -Wno-parentheses -Wno-logical-op-parentheses -Wno-pointer-bool-conversion -Wno-unused-value -Wno-unsequenced -Wno-constant-logical-operand -Wno-undef-prefix")
   endif()
 
   if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
