speedup appveyor build

As the "new buildbot" consistently fails to provide a working compile environment on Windows, and the "old buildbot" machines are overloaded, there is a need to keep to build Appveyor build relatively fast, especially because github shows a "badge" for it, so people would know if it failed or now.

The builds have slowed down from 20 min on 10.2 to 40 min on 10.7, it seems