Actually check if the file exists rather than assume it doesn't

Created originally on Bitbucket by dstufft (Donald Stufft)

Was already merged in Bitbucket before import, marked as merged by the import user

Source changeset not longer availableat the time of import. Commit listwill look empty

You can test this out by getting the cryptography code, and running python setup.py build_ext twice without this PR, it will compile two times even though nothing has changed. If you then apply this patch to cffi and then delete the build dir and run python setup.py build_ext twice again it'll only build it once.