PyPy fixes

This patch includes several fixes related to PyPy that I think is worth landing in the master branch.

1. PyPy's numpy does not and is not planning to support object. This is fine (since pyopencl does not support that either) but need to be detected.
2. the vector in opencl.array triggers another `NotImplementedError`. I'm not familiar with what's going on there but you are right that disabling it does not seem to cause other problems.
3. Fix a test when building opencl with old opencl headers. It still does not help with the crash if pocl icd is loaded but should be good to have anyway....

Along with the poll request on the `cffi` branch, all the import has no error now. However, I am getting errors like
```python
Traceback (most recent call last):
  File "app_main.py", line 75, in run_toplevel
  File "demo_elementwise.py", line 23, in <module>
    lin_comb(5, a_gpu, 6, b_gpu, c_gpu)
  File "/opt/pypy/site-packages/pyopencl/elementwise.py", line 307, in __call__
    kernel.set_args(*invocation_args)
  File "/opt/pypy/site-packages/pyopencl/__init__.py", line 546, in kernel_set_args
    % (i+1, str(e), advice))
RuntimeError: maximum recursion depth exceeded
```
in `set_args` and I'm not sure what's happening yet....