configure: use cffi.FFI().verify() to test buildability of CFFI bindings
Checking existence of pyconfig.h to determine whether CFFI-based
notmuch bindings are buildable is not enough; for example Fedora 32
ships pyconfig.h in python3-libs package, but python3-devel is required
to be installed for the bindings to build.
Executing cffi.FFI().verify() is pretty close to what is done in
bindings/python-cffi/notmuch2/_build.py to get the c code part of the
bindings built.
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