Remove the need to include C++ <exception>
Remove the need to include C++ <exception>

We were only doing it to get two definitions, which are both mandated by
the spec, so we may as well avoid all of the headaches involved.

Also remove the extern "C" from the runtime.h include while we're here,
as it is no longer needed.

Fixes #69

gnustep/libobjc2,gnustep/libobjc2