[cffi] Allow generated shared library to have a name other than native.so

[timkpaine]

When using the cffi backend, the generated shared library is always named native.so. It would be good if this:

• was configureable or
• matched the name of the library (e.g. my_extension would generated my_extension.so or libmy_extension.so)

[messense]

When using the cffi backend, the generated shared library is always named native.so

Is there any real issues caused by the hard-coded name?

[timkpaine]

@messense linking against multiple libraries generated in this fashion (which i plan on doing)