Cython modules not automatically rebuild through setup.py on changes of pxd files

After some frustrating hours I discovered that if you use a distutils setup.py script to create a cython module the module is not updated if you only change a pxd header file. Especially in a bigger project with many cython modules this can be easily forgotten, so in case of unexpected behaviour just try removing all pyd files so everything will be build fresh.

It is perhaps even saver to touch all .pyx files because if you remove the pyd file the .c file is not necessarily recreated!