Fix pangocffi to not run verify

In light of [1], try to get pangocffi to use `ffi.dlopen()` rather than `ffi.verify()`. There is still some problems with the tests, I probably need to edit the cdef to not have ellipses (???). It'd be nice if we could run `verify`, but it'd take an upstream change.

[1] https://github.com/SimonSapin/cairocffi/pull/37#issuecomment-47490660