Explain setuptools build dir cleaning

Put bdist_wheel after build_ext too, which seems more logical, though
both variants work.

build-wheels.bat

@@ -42,7 +42,9 @@ if -%1-==-- goto end
 
 echo Building wheel for Python %1
 
-py -%1 setup.py clean --all bdist_wheel build_ext
+:: Clean the setuptools build directory before building the wheel, to make sure
+:: we build from scratch
+py -%1 setup.py clean --all build_ext bdist_wheel
 if %errorlevel% neq 0 (
     echo Check that you have the 'wheel' module installed for Python %1 and are
     echo building with the right compiler