avoid setting Py_LIMITED_API when py_limited_api=False (#106)

* avoid setting Py_LIMITED_API when py_limited_api=False

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

* only unset limited api for explicit py_limited_api=False

src/cffi/setuptools_ext.py

@@ -101,6 +101,11 @@ def _set_py_limited_api(Extension, kwds):
             # try to set 'py_limited_api' anyway.  At worst, we get a
             # warning.
             kwds['py_limited_api'] = True
+
+    if kwds.get('py_limited_api') is False:
+        # avoid setting Py_LIMITED_API if py_limited_api=False
+        # which _cffi_include.h does unless _CFFI_NO_LIMITED_API is defined
+        kwds.setdefault("define_macros", []).append(("_CFFI_NO_LIMITED_API", None))
     return kwds
 
 def _add_c_module(dist, ffi, module_name, source, source_extension, kwds):