setup.py: implicitly enable --cython when running from git

CONTRIBUTE.rst

@@ -75,13 +75,10 @@ A useful trick for working on the source code is to build in-place so that there
 is no need to re-install after each change.  This can be done with the following
 command ::
 
-    python setup.py build_ext -i --cython
+    python setup.py build_ext -i
 
 The ``kwant`` subdirectory of the source distribution will be thus turned into
 a proper Python package that can be imported.  To be able to import Kwant from
 within Python, one can either work in the root directory of the distribution
 (where the subdirectory ``kwant`` is located), or make a (symbolic) link from
 somewhere in the Python search path to the the package subdirectory.
-
-The option ``--cython`` enables the translation of .pyx files into .c files.
-It is only needed if any .pyx files have been modified.

setup.py

@@ -62,7 +62,7 @@ try:
     sys.argv.remove(CYTHON_OPTION)
     use_cython = True
 except ValueError:
-    use_cython = False
+    use_cython = version_is_from_git
 
 if use_cython:
     try: