just a trivial comment:
maybe its docstring should read:
"True if DEFCFUN is marked to be inlined in the lexical environment ENV."
also, in my own projects i'd call it something like
%inline-defcfun?
but in CFFI i guess it should be something like
%inline-defcfun-p
unless you have plans to use it in a more general way. then maybe
%inline-cffi-definitions-p