Find out which architecture we are running on

The architecure and operating system the engine is compiled for determines whether we can use or not certain libraries. This script, taken from a PVM distribution, uses a heuristic (which may need to be tuned from time to time) to find out the platform. It returns a string which is used throughout the engine (in #ifdefs) to enable/disable certain characteristics.

   Usage: ciao_get_arch