This uses one global event handler function on the C level, as
enforced by CFFI, and caters for registering a per-context event
handeler. A simple fixture for doing this during tests is also
implemented.
This cleans up the build somewhat:
- Move time_t declaration to _build.py
- Uses Python's distutils to invoke the "correct" compiler. Or at
least does not make this code responsible to picking a compiler
- Uses the compiler to find the installed header file, builds
against this installed version. Since linking will also be
against this installed version this seems to make some sense.
This seems somewhat ugly though.
