setup.py: run cython if core.c does not exist

setup.py

@@ -40,12 +40,16 @@ class my_build_ext(build_ext.build_ext):
     def compile_cython(self):
         sources = glob.glob('gevent/*.pyx') + sorted(glob.glob('gevent/*.pxi'))
         if not sources:
-            print >> sys.stderr, 'Could not find gevent.core sources'
-            return
+            if not os.path.exists('gevent/core.c'):
+                print >> sys.stderr, 'Could not find gevent/core.c'
+        if os.path.exists('gevent/core.c'):
             core_c_mtime = os.stat('gevent/core.c').st_mtime
             changed = [filename for filename in sources if (os.stat(filename).st_mtime - core_c_mtime) > 1]
-        if changed:
+            if not changed:
+                return
             print >> sys.stderr, 'Running cython (changed: %s)' % ', '.join(changed)
+        else:
+            print >> sys.stderr, 'Running cython'
         cython_result = os.system('cython gevent/core.pyx')
         if cython_result:
             if os.system('cython -V 2> %s' % os.devnull):