don't fail if trying to mkdir when the dir has been created in the meantime

When ninja tries to do the equivalent of "mkdir -p", it will fail if another process (or possibly another ninja task) creates one of the intermediate dirs in the meantime.  This can be quite annoying, but it's fairly simple to catch.

NinjaMain::EnsureBuildDirExists doesn't fail in case of EEXIST (the check would be redundant if this patch is landed), but Builder::StartEdge does.