What triggers the passing of -imultilib to a language driver?
It is passed together with -isystem, -iprefix and friends when %I is in a spec. I'm sure you can define a new spec function and use it to pass the multilib_dir variable down to the Ada driver (see default_compilers, I guess you have to read some gcc.c).
Once gcc passes this info to gnat1 it will likely be easy to have gnatmake/bind/link extract it when needed since those tools call gcc.
I believe you on this. :-) Paolo
