Debugging

Use the -DS command-line option to turn on debugging of the
multi-threading code. Under Linux, that also turns on a quick
hack I did to grab a bit of extra information from segfaults.
If you have a fancier gdb/threads setup than I do then you'll
have to delete the lines in perl.c which say
    #if defined(DEBUGGING) && defined(USE_THREADS) && defined(__linux__)
        DEBUG_S(signal(SIGSEGV, (void(*)(int))catch_sigsegv););
    #endif