I think this is why the Travis-CI builds are hanging

We're hanging trying to output some diagnostic information if we die at the wrong point.
This doesn't address the underlying issue, but at least we'll start getting useful
results from Travis-CI.

It looks like the underlying issue (libunwind can't find the interpreter function)
only happens with the cmake build, and I can't tell why yet.

src/runtime/stacktrace.cpp

@@ -142,6 +142,13 @@ extern "C" void abort() {
 
         fprintf(stderr, "Someone called abort!\n");
 
+        // If we call abort(), things may be seriously wrong.  Set an alarm() to
+        // try to handle cases that we would just hang.
+        // (Ex if we abort() from a static constructor, and _printStackTrace uses
+        // that object, _printStackTrace will hang waiting for the first construction
+        // to finish.)
+        alarm(1);
+
         _printStacktrace();
     }