Add a hack to the fatal signal handlers to invoke gdb and get a full stack
trace on crash.  This won't fix anything, but by activating this mode
in the testsuite, we can get more detail on the alpha and sparc FTBFSes.

Index: monotone-1.0/src/unix/main.cc
===================================================================
--- monotone-1.0.orig/src/unix/main.cc	2009-04-12 13:39:49.000000000 -0700
+++ monotone-1.0/src/unix/main.cc	2009-04-12 13:42:56.000000000 -0700
@@ -36,12 +36,21 @@
 
 #include "../base.hh"
 #include <signal.h>
-#include <time.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/resource.h>
 #include <unistd.h>
+#include <time.h>
+
+#if __GLIBC__ >= 3 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 1)
+# define HAVE_BACKTRACE
+# include <execinfo.h>
+#endif
 
 static char const * argv0;
+#ifdef HAVE_BACKTRACE
+static bool backtrace_on_crash = false;
+#endif
 
 // a convenient wrapper
 inline void
@@ -75,6 +84,18 @@
   write_str_to_stderr("do not send a core dump, but if you have one, "
                       "\nplease preserve it in case we ask you for "
                       "information from it.\n");
+#ifdef HAVE_BACKTRACE
+  if (backtrace_on_crash)
+    {
+      void *tracebuf[20];
+      int frames = backtrace(tracebuf, 20);
+      if (frames)
+        {
+          write_str_to_stderr("stack trace:\n");
+          backtrace_symbols_fd(tracebuf, frames, 2);
+        }
+    }
+#endif
 
   raise(signo);
   // The signal has been reset to the default handler by SA_RESETHAND
@@ -129,6 +150,12 @@
 
   argv0 = argv[0];
 
+#ifdef HAVE_BACKTRACE
+  char *ev = getenv("MTN_STACKTRACE_ON_CRASH");
+  if (ev && ev[0])
+    backtrace_on_crash = true;
+#endif
+
   bug_signal_action.sa_flags   = SA_RESETHAND;
   bug_signal_action.sa_handler = &bug_signal;
   sigemptyset(&bug_signal_action.sa_mask);