cffi: support building against 1.5.0
cffi: support building against 1.5.0

The header file layout changed. So we had to reflect that.

In addition, the deprecated __attribute__ syntax changed slightly. So
we had to adopt our preprocessor normalizer accordingly to prevent
the cdef parser from being confused.

As of this commit, the cffi backend passes all tests against 1.5.0.

indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard