During the upgrade from setuptools 23.0.0 to 25.1.6, it seems quoting of arguments in Windows command lines got broken (specifically when linking C extensions), which you can witness on those two AppVeyor builds:

• ok with 23.0.0: https://ci.appveyor.com/project/pitrou/numba/build/1.0.1220#L185
• fail with 25.1.6: https://ci.appveyor.com/project/pitrou/numba/build/1.0.1227#L185

This seems to be a setuptools issue since it's the only piece of the environment that changed between the two builds.