Don't use functions/macros not defined in the libgap API

[E. M. Bray]

Before there was an effort to strictly define a "public" C API for GAP, Sage's libgap interface used anything it could find from GAP's headers.

In most cases I believe the (still somewhat informal) public API should meet our needs; in cases where it does not it might indicate some surface area where the API needs to be expanded. There might also still be some cases where it is more efficient to go around the API and use GAP internals directly (though that too should be discussed).