An extension should not be necessary. From an Ada perspective it is
supposed to work with pragma Import (C, certainly if Ada and C are from
the same vendor.
The Ada 95 LRM Annex B.3 Interfacing with C has a specific reminder on
handling variadic calls:
(12) A C function that takes a variable number of arguments can
correspond to several Ada subprograms, taking various specific numbers
and types of parameters.
The above is a note, which has no normative force whatsoever. Furthermore.
it says "can" which in standardeese means that this is an allowable
implementation. In some architectures, the calling sequences are radically
different, so it would be impsosible to "follow" the suggestion of this
note.
So, the LRM is saying that the proper approach is to use overloaded Ada
subprograms to call variadic C routines.
It is not saying that at all!