pyx.build: Extension.depends += sync.pxd, time.pxd

Make built extensions depend on golang/{sync,time}.pxd and rebuild if
those files change. A bit overkill to depend globally, but more correct,
since an extension that is using pygolang can be using those packages.

Before the patch - without those depends - since distutils does not
implement proper dependency tracking, extensions - even those that
actually depend on sync/time pxd, were not rebuilt.

sync.pxd was missed to be added in 34b7a1f4 (golang: Expose Sema and
Mutex as public Python and Cython/nogil API). time.pxd was missed to be
added in ce8152a2 (pyx api: Provide sleep).

golang/pyx/build.py

@@ -166,6 +166,10 @@ def Extension(name, sources, **kw):
     dependv.append('%s/golang/libgolang.h'  % pygo)
     dependv.append('%s/golang/_golang.pxd'  % pygo)
     dependv.append('%s/golang/__init__.pxd' % pygo)
+    dependv.append('%s/golang/sync.pxd'     % pygo)
+    dependv.append('%s/golang/_sync.pxd'    % pygo)
+    dependv.append('%s/golang/time.pxd'     % pygo)
+    dependv.append('%s/golang/_time.pxd'    % pygo)
     kw['depends'] = dependv
 
     # workaround pip bug that for virtualenv case headers are installed into