#include IANAL.c
If I understand what you are doing, I don't think your in violation but your really pushing the edge.
I would consider the deciding question to be how much your package depends on transmogrify. If your program can run and still function usefully then you should be OK.
The same problem comes up with executable programs. The opinion of the programmers of Linux in general is that just making a call to a kernel function doesn't make your program derived from the kernel. But you can't work around the GPL by building your program as a shell around GPL code. Your program has to have independent usefull functionality.
The idea is fairly simple, but where to draw the line in practice is open to debate and has never been decided in court.
It would be far more acceptable to figure out how to use ANTLR directly. Otherwise you leave yourself open to problems down the road.
Jay