[Patch/C++]: Use dllonexit for static destructors on mingw32

[Danny Smith]

> From: Benjamin Kosnik
> Sent: Tuesday, 31 October 2006 10:55 p.m.
> 
> Yay. This will make a big difference in QoI of C++ on mingw32.
> 
> You should check out { dg-require-cxa-atexit "" } testcases, 
> and see if
> this helps. I know that libstdc++ (and I think others) have this.
> 
> testsuite/ext/mt_allocator/deallocate_local_thread-5.cc
> testsuite/ext/mt_allocator/deallocate_local_thread-7.cc
> testsuite/ext/mt_allocator/deallocate_local-6.cc
> testsuite/ext/mt_allocator/deallocate_local-8.cc

Actually, those  tests passed   (when run manually) before the patch.
For awhile
had given me a false sense that there were no major problems with the
way destructors were run.
What did cause problems was using  libstdc++ as dll. 

Danny


> 
> -benjamin