fixes in setup.py to compile in empty environment

    1. Now can be compiled without MUMPS
       (previously exception was thrown)
    2. Gives sensible error message for Git install,
       but without installed Cython.

setup.py

@@ -12,6 +12,7 @@ from __future__ import print_function
 
 import sys
 
+
 def ensure_python(required_version):
     v = sys.version_info
     if v[:3] < required_version:
@@ -32,9 +33,7 @@ import subprocess
 import configparser
 import collections
 from setuptools import setup, find_packages, Extension, Command
-from sysconfig import get_platform
-from distutils.errors import DistutilsError, DistutilsModuleError, \
-    CCompilerError
+from distutils.errors import DistutilsError, CCompilerError
 from distutils.command.build import build
 from setuptools.command.sdist import sdist
 from setuptools.command.build_ext import build_ext
@@ -163,6 +162,7 @@ def init_cython():
     global cythonize, cython_help
 
     cython_option = '--cython'
+    required_cython_version = (0, 22)
     try:
         sys.argv.remove(cython_option)
         cythonize = True
@@ -186,15 +186,14 @@ def init_cython():
                 cython_version[-1] -= 1
             cython_version = tuple(cython_version)
 
-            required_cython_version = (0, 22)
             if cython_version < required_cython_version:
                 cythonize = None
 
-            if cythonize is None:
-                msg = ("Install Cython >= {0} or use"
-                       " a source distribution (tarball) of Kwant.")
-                ver = '.'.join(str(e) for e in required_cython_version)
-                cython_help = msg.format(ver)
+        if cythonize is None:
+            msg = ("Install Cython >= {0} or use"
+                    " a source distribution (tarball) of Kwant.")
+            ver = '.'.join(str(e) for e in required_cython_version)
+            cython_help = msg.format(ver)
     else:
         msg = "Run setup.py with the {} option to enable Cython."
         cython_help = msg.format(cython_option)
@@ -411,7 +410,7 @@ def configure_special_extensions(exts, build_summary):
             build_summary.append('Auto-configured MUMPS')
         else:
             mumps = None
-            del exts['mumps']
+            del exts['kwant.linalg._mumps']
             build_summary.append('No MUMPS support')
 
     if mumps: