[mlir][python] declare _PyClassMethod_New undefined at link time (llvm#121597)

`NanobindAdaptors.h` uses `PyClassMethod_New` to build `pure_subclass`es
but nanobind doesn't declare this API as undefined in its linker flags.
So we need to declare it as such for downstream users that do not do
something like `-undefined dynamic_lookup`

Author: makslevental

mlir/cmake/modules/AddMLIRPython.cmake

@@ -683,6 +683,13 @@ function(add_mlir_python_extension libname extname)
           ${eh_rtti_enable}
       )
     endif()
+    
+    if(APPLE)
+      # NanobindAdaptors.h uses PyClassMethod_New to build `pure_subclass`es but nanobind
+      # doesn't declare this API as undefined in its linker flags. So we need to declare it as such
+      # for downstream users that do not do something like `-undefined dynamic_lookup`.
+      set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} -Wl,-U -Wl,_PyClassMethod_New")
+    endif()
   endif()
 
   target_compile_options(${libname} PRIVATE ${eh_rtti_enable})