I'd like to add a C extension to speed up a small bit of code in a package (Tornado), but make it optional both for compatibility with non-cpython implementations and for ease of installation on systems without a C compiler available.  Ideally any user who runs "pip install tornado" on a system capable of compiling extensions would get the extensions; if this capability cannot be detected automatically I'd prefer the opt-out case to be the one that requires non-default arguments.  Are there any packages that provide a good example to follow for this?