Latest known-good SPIRV-Tools: WARNING: Needs python 3.x.

You may need to install python 3.x and also
[windows users] may need to get it in your path in front of python 2.7.x.

If you don't need the external projects (i.e., SPIRV-Tools) and don't want
to upgrade to python 3, don't create the External subdirectory and CMake
won't require python 3.  (CMake has a bug that makes it find the wrong one
on a first pass if the top level does not require the version needed by
lower levels.)

.appveyor.yml

@@ -32,6 +32,7 @@ matrix:
 # scripts that run after cloning repository
 install:
   - C:/Python27/python.exe update_glslang_sources.py
+  - set PATH=C:\ninja;C:\Python36;%PATH%
   - git clone https://github.com/google/googletest.git External/googletest
   - cd External/googletest
   - git checkout 440527a61e1c91188195f7de212c63c77e8f0a45