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.)

CMakeLists.txt

@@ -114,6 +114,12 @@ function(glslang_set_link_args TARGET)
     endif()
 endfunction(glslang_set_link_args)
 
+# CMake needs to find the right version of python, right from the beginning,
+# otherwise, it will find the wrong version and fail later
+if(IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/External)
+    find_package(PythonInterp 3 REQUIRED)
+endif()
+
 # We depend on these for later projects, so they should come first.
 add_subdirectory(External)