ada/6767: make gnatlib-shared fails on -laddr2line

[Ludovic Brenta]

I've managed to bootstrap 3.3 on another machine and the problem is
gone.

I traced the change to Makefile.in revision 1.22 (2002-03-08) when
"ro" merged in changes from ACT.  Now, the GNAT libraries (either
static or shared) no longer include the file g-trasym.o (package
GNAT.Traceback.Symbolic), so the feature is unavailable, but "make
gnatlib-shared" succeeds.

There is still a message "ld: cannot find -laddr2line" when trying to
link a program that uses GNAT.Traceback.Symbolic, but that is
expected.

--
Ludovic Brenta.