external: update backward-cpp to v1.4

rename the directory name to backward-cpp

Closes: #803
Signed-off-by: Shengjing Zhu <i@zhsj.me>

external/backward-cpp/test_package/main.cpp

@@ -0,0 +1,60 @@
+#include <backward/backward.hpp>
+#include <iostream>
+#include <stdexcept>
+#include <sstream>
+
+using namespace backward;
+
+class TracedException : public std::runtime_error
+{
+public:
+    TracedException() :
+        std::runtime_error(_get_trace())
+    {}
+
+private:
+    std::string _get_trace()
+    {
+        std::ostringstream ss;
+
+        StackTrace stackTrace;
+        TraceResolver resolver;
+        stackTrace.load_here();
+        resolver.load_stacktrace(stackTrace);
+
+        for(std::size_t i = 0; i < stackTrace.size(); ++i)
+        {
+            const ResolvedTrace trace = resolver.resolve(stackTrace[i]);
+
+            ss << "#" << i << " at " << trace.object_function << "\n";
+        }
+
+        return ss.str();
+    }
+};
+
+void f(int i)
+{
+    if(i >= 42)
+    {
+        throw TracedException();
+    }
+    else
+    {
+        std::cout << "i=" << i << "\n";
+        f(i + 1);
+    }
+}
+
+int main()
+{
+    try
+    {
+        f(0);
+    } catch (const TracedException& ex)
+    {
+        std::cout << ex.what();
+    }
+}
+
+