This is a C library wich also presents a python interface.  It can be 
used just as well without python, so I've made python only a native 
input, not a regular input.  There's currently a non-determinism 
issue for the python interface, related to 
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=22533

I also tried to automatically generate the string 
“lib/python3.4/site-packages”, so it's updated automatically when the 
python version changes, but didn't manage to do that (for example I 
tried

   (version-major+minor (package-version python))

but the package variable “python” is not available when the build 
expression is turned into a derivation, as far as I understand it?). 
Advice welcome.

Thomas