setup.py: disable cython by default

The pip tool messes up the timestamps of files, so with our previous strategy
of enabling cython by default the build fails if Cython is not available.

Also, turn the "warning" into a "caution" message that is less frightening and
send it to stdout only, so that it does not appear with pip.

CONTRIBUTE.rst

@@ -75,10 +75,13 @@ 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
+    python setup.py build_ext -i --cython
 
 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

@@ -38,7 +38,7 @@ MANIFEST_IN_FILE = 'MANIFEST.in'
 README_END_BEFORE = 'See also in this directory:'
 STATIC_VERSION_PATH = ('kwant', '_kwant_version.py')
 REQUIRED_CYTHON_VERSION = (0, 22)
-NO_CYTHON_OPTION = '--no-cython'
+CYTHON_OPTION = '--cython'
 TUT_DIR = 'tutorial'
 TUT_GLOB = 'doc/source/tutorial/*.py'
 TUT_HIDDEN_PREFIX = '#HIDDEN'
@@ -59,10 +59,10 @@ version = _common.version
 version_is_from_git = _common.version_is_from_git
 
 try:
-    sys.argv.remove(NO_CYTHON_OPTION)
-    use_cython = False
-except ValueError:
+    sys.argv.remove(CYTHON_OPTION)
     use_cython = True
+except ValueError:
+    use_cython = False
 
 if use_cython:
     try:
@@ -424,7 +424,7 @@ def ext_modules(extensions):
     if problematic_files:
         problematic_files = ", ".join(problematic_files)
         msg = ("Some Cython source files are newer than files that should have\n"
-               " been derived from them, but {}.\n"
+               "been derived from them, but {}.\n"
                "\n"
                "Affected files: {}")
         if use_cython:
@@ -438,8 +438,11 @@ def ext_modules(extensions):
             complain_cython_unavailable()
             exit(1)
         else:
-            reason = "the option --no-cython has been given"
-            print(banner(" Warning "), msg.format(reason, problematic_files),
+            reason = "the option {} has not been given".format(CYTHON_OPTION)
+            dontworry = ('(Do not worry about this if you are building Kwant\n'
+                         'from unmodified sources, e.g. with "pip install".)\n')
+            print(banner(" Caution "), dontworry,
+                  msg.format(reason, problematic_files),
                   banner(), sep='\n', file=sys.stderr)
 
     return result