controlling exception type per target

[Jason Merrill]

Sorry I haven't gotten back to you before.  I think this should be handled
by setting up exceptions_via_longjmp in main(), with a simple

#ifdef DWARF2_UNWIND_INFO
exceptions_via_longjmp = DWARF2_UNWIND_INFO;
#endif

> Should I expand this scheme on into crtstuff as well?

You can tell whether you are doing COFF or ELF with #ifdefs, right?  I
would think you could special-case the SCO tm.h so that when CRT_BEGIN or
CRT_END are defined, you use unconditional defines.  irix6.h does something
similar for n32/n64 multilibbing.

Jason