Revert "Revert "Build Cython in parallel on appveyor if distutils support it.""

This reverts commit 4900109c.
This is trying to parallelise the appveyor build again after allowing the setup.py script to be imported on a multiprocessing "spawn".

appveyor.yml

@@ -8,17 +8,20 @@ environment:
     # See: https://stackoverflow.com/questions/11267463/compiling-python-modules-on-windows-x64/13751649#13751649
     WITH_ENV: "cmd /E:ON /V:ON /C .\\appveyor\\run_with_env.cmd"
     BACKEND: c
+    PARALLEL: "-j4"
 
   matrix:
     - PYTHON: "C:\\Python27"
       PYTHON_VERSION: "2.7"
       PYTHON_ARCH: "32"
       PYTHONIOENCODING: "utf-8"
+      PARALLEL: ""
 
     - PYTHON: "C:\\Python27-x64"
       PYTHON_VERSION: "2.7"
       PYTHON_ARCH: "64"
       PYTHONIOENCODING: "utf-8"
+      PARALLEL: ""
 
     - PYTHON: "C:\\Python38"
       PYTHON_VERSION: "3.8"
@@ -60,16 +63,19 @@ environment:
     - PYTHON: "C:\\Python34"
       PYTHON_VERSION: "3.4"
       PYTHON_ARCH: "32"
+      PARALLEL: ""
 
     - PYTHON: "C:\\Python34-x64"
       PYTHON_VERSION: "3.4"
       PYTHON_ARCH: "64"
+      PARALLEL: ""
 
     - PYTHON: "C:\\Python27-x64"
       PYTHON_VERSION: "2.7"
       PYTHON_ARCH: "64"
       BACKEND: cpp
       PYTHONIOENCODING: "utf-8"
+      PARALLEL: ""
 
 clone_depth: 5
 
@@ -89,7 +95,7 @@ install:
 
 build: off
 build_script:
-  - "%WITH_ENV% %PYTHON%\\python.exe setup.py build_ext --inplace"
+  - "%WITH_ENV% %PYTHON%\\python.exe setup.py build_ext --inplace %PARALLEL%"
   - "%WITH_ENV% %PYTHON%\\python.exe setup.py bdist_wheel"
 
 test: off