I didn't know that there are build and runtime versions. If the runtime
version is available, it should be preferred.

Hum, maybe its better to expose the __system_property_get() function in the
os module, something similar to os.confstr()?
https://docs.python.org/dev/library/os.html#os.confstr

If the OS returns a string, I suggest to return a string in Python too. The
caller would be responsible to convert it to integer. What do you think of
that?