Backport of _Qdmodule.c 1.8, qdsupport.py 1.38:

Backport of _Qdmodule.c 1.8, qdsupport.py 1.38:
Weaklink most toolbox modules, improving backward compatibility. Modules will no longer fail to load if a single routine is missing on the curent OS version, in stead calling the missing routine will raise an exception.

Should finally fix 531398. 2.2.1 candidate.

Also blacklisted some constants with definitions that were not Python-compatible.