avoid setting Py_LIMITED_API when py_limited_api=False

[minrk]

previously, setting py_limited_api=False only appeared to affect the ABI tag of modules, as Py_LIMITED_API was still unconditionally defined in most cases by _cffi_include.h here.

The changlog says:

You can manually disable Py_LIMITED_API by calling ffi.set_source(..., py_limited_api=False).

but this doesn't appear to have been the case. With this change, the behavior matches at least my understanding of the docs on the matter.

The fact that setting py_limited_api=False didn't work is what led me to #104, which is unnecessary with this PR.

[arigo]

The setting py_limited_api=False in the comment probably worked at some point for some goal like running Windows with CPython 3.4. I'm not sure I understand all possible consequences of this small patch, though, so maybe play it safer and only define _CFFI_NO_LIMITED_API if py_limited_api is explicitly set to False, and not if it is simply missing?

[minrk]

Makes perfect sense; done!

[arigo]

Looks good, thanks!