[libobjc] Make exception.c safe for unwind.h

[Tom Tromey]

>>>>> "Andreas" == Andreas Schwab <schwab@suse.de> writes:

Andreas> unwind.h is also used by libjava.

Andreas> 2005-05-19  Andreas Schwab  <schwab@suse.de>
Andreas> 	* configure.ac (GCC_UNWIND_INCLUDE): Add
Andreas> 	`-I$(libgcj_basedir)/../include'.
Andreas> 	* configure: Regenerate.
Andreas> 	* include/java-stack.h: Include <tsystem.h>.

Why not just do what libjava/exception.cc does?

#define gcc_unreachable() ::abort ()

Tom