[patch ada]: Fix issues about multilib build of native windows and handle MSG_WAITALL for windows native targets

[Arnaud Charlet]

> this patch fixes some nits about MSG_WAITALL for mingw(-w64) targets.
> Additionally it improves multilib configury for windows native
> targets.

Can you please split this patch in two, since the MSG_WAITALL is one thing,
and the multilib is a completely different patch.

Also, can you explain the setting of SO_LIB in more details?

Concerning the setting of EH_MECHANISM, it's time to set ZCX to True by
default on Windows 32bits as well, and set EH_MECHANISM=-gcc on all
Windows variants IMO.

Arno