rustc.exe depends on winpthreads

[klutzy]

Buildbot seems to use mingw-w64 package from mingw-builds with "thread=posix" option. In this setting, libstdc++-6.dll depends on libwinpthread-1.dll. Thus llvm binaries and rustc.exe/rustdoc.exe depend on it.
The dependency can be dropped by re-installing mingw-builds with "thread=win32" option.

cc @brson @vadimcn

[klutzy]

Side issue: stage0 doesn't have libwinpthread-1.dll now. It causes missing dll error on old mingw setup.

[qoh]

Using the nightly installer, running rustc always gives "The program can't start because libwinpthread-1.dll is missing from your computer. Try reinstalling the program to fix this problem." for me (can't run at all).

[vadimcn]

Oh, that's right, it's coming back to me now: there is a performance bug in winpthreads. I agree, we should use threads=win32.

[brson]

I've started updating the bot to use win32 threads.

[brson]

I just updated the bots to use win32 threads.

[brson]

We have a new snap based on win32 threads. Can anyone confirm this is ok now?

[klutzy]

Yep, it's fixed!

[tomaka]

This issue is back in the 64 bits windows nightlies.
Starting rustc immediatly pops a missing libwinpthreads-1.dll error.
The 32 bits nightlies are working.

[vadimcn]

Yep. Only seems to be a problem with nightlies though, snapshots are OK.
@brson, perhaps the nightly bot got installed with "posix" mingw?