Various appveyor tweaks
Various appveyor tweaks

* Improved build parameterization with environment variables
* Split external lib build into separate pre-build script
* Fix bug in ctest invocation

rotorliu/displaz,JoshChristie/displaz,rotorliu/displaz,JoshChristie/displaz,rotorliu/displaz,JoshChristie/displaz,rotorliu/displaz,JoshChristie/displaz,JoshChristie/displaz