Our Appveyor instance was recently changed to by default use a VM that only contains VS2017 for libcc builds. That's fine for master and 3-0-x, but older versions need VS2017, so this PR is an Appveyor override to use VS2015 instead of the default of VS2017
Our Appveyor instance was recently changed to by default use a VM that only contains VS2017 for libcc builds. That's fine for master and 3-0-x, but older versions need VS2017, so this PR is an Appveyor override to use VS2015 instead of the default of VS2017