It is now impossible to create a native C/C++ library and export C functions that can be used by Flash commands for instance. The sample C-level extensibility project "SampleDll" now does not work in Flash Pro CC as well:
http://help.adobe.com/en_US/flash/cs/extend/WS5b3ccc516d4fbf351e63e3d1 18a9024f3f-7783CS5.html
http://download.macromedia.com/pub/developer/flash/flash-pro-samples.z ip
When Flash Pro CC is installed the "<user_home_dir>/AppData/Local/Adobe/Flash CC/<language_code>/Configuration/External Libraries" already contains a dll that exports the MM_Init and MM_Terminate, so presumably the C-level extensibility is still available, but its probable new usage might not be yet documented.