setup.py: add "build_ext --cython=" option, which allows users to specify the...

setup.py: add "build_ext --cython=" option, which allows users to specify the path to the cython executable.

this also allows users to skip automatic cython compilation completely
by specifying an empty cython executable, e.g. by providing the
following setup.cfg:

,----[ setup.cfg ]
| [build_ext]
| cython=
`----

I use this to manage a copy of gevent inside another git repository
while at the same time making sure that core.c doesn't get
regenerated.

setup.py

@@ -41,6 +41,12 @@ libraries = []
 
 
 class my_build_ext(build_ext.build_ext):
+    user_options = (build_ext.build_ext.user_options
+                    + [("cython=", None, "path to the cython executable")])
+
+    def initialize_options(self):
+        build_ext.build_ext.initialize_options(self)
+        self.cython = "cython"
 
     def compile_cython(self):
         sources = glob.glob('gevent/*.pyx') + glob.glob('gevent/*.pxi')
@@ -52,12 +58,12 @@ class my_build_ext(build_ext.build_ext):
             changed = [filename for filename in sources if (os.stat(filename).st_mtime - core_c_mtime) > 1]
             if not changed:
                 return
-            print >> sys.stderr, 'Running cython (changed: %s)' % ', '.join(changed)
+            print >> sys.stderr, 'Running %s (changed: %s)' % (self.cython, ', '.join(changed))
         else:
-            print >> sys.stderr, 'Running cython'
-        cython_result = os.system('cython gevent/core.pyx')
+            print >> sys.stderr, 'Running %s' % self.cython
+        cython_result = os.system('%s gevent/core.pyx' % self.cython)
         if cython_result:
-            if os.system('cython -V 2> %s' % os.devnull):
+            if os.system('%s -V 2> %s' % (self.cython, os.devnull)):
                 # there's no cython in the system
                 print >> sys.stderr, 'No cython found, cannot rebuild core.c'
                 return
@@ -65,8 +71,8 @@ class my_build_ext(build_ext.build_ext):
 
     def build_extension(self, ext):
         compile_libevent(self)
-
-        self.compile_cython()
+        if self.cython:
+            self.compile_cython()
         result = build_ext.build_ext.build_extension(self, ext)
         # hack: create a symlink from build/../core.so to gevent/core.so
         # to prevent "ImportError: cannot import name core" failures