Setup uses cython even if WITH_CYTHON is False
Currently, if setup.py is called with the environment variable WITH_CYTHON set to
- True : setup doesn't use cython, the pre-generated C files are used.
- False : setup uses cython

Judging by the code (the other lines all set WITH_CYTHON to False when not using cython) this should probably be the opposite.

diff --git a/setup.py b/setup.py
index d859e4f71..66537a23f 100644
--- a/setup.py
+++ b/setup.py
@@ -268,7 +268,9 @@ def check_cython():
     """
 
     if "WITH_CYTHON" in os.environ:
-        return os.environ["WITH_CYTHON"] == "False"
+        if os.environ["WITH_CYTHON"] in ["False", "0", 0]:
+            os.environ["WITH_CYTHON"] = "False"
+            return False
 
     if "--no-cython" in sys.argv:
         sys.argv.remove("--no-cython")
['setup.py']