gcd: allow user to override (some of) heuristics.

GiNaC tries to avoid expanding expressions while computing GCDs and applies
a number of heuristics. Usually this improves performance, but in some cases
it doesn't. Allow user to switch off heuristics.

Part 1:

* add new (optional) argument to gcd() to control its behaviour.
* introduce gcd_options structure.

N.B. No actual code changes so far, the actual handling of newly introduced
options is the subject of further patches.