fix a bug of defunct process by adding a SIGCHLD handler.

subprocess.hpp

@@ -42,6 +42,7 @@ Documentation for C++ subprocessing libraray.
 #include <cassert>
 #include <cstring>
 #include <cstdio>
+#include <csignal>
 #include <future>
 #include <vector>
 #include <sstream>
@@ -1150,6 +1151,11 @@ void Popen::execute_process() throw (CalledProcessError, OSError)
   }
   exe_name_ = vargs_[0];
 
+  std::signal(SIGCHLD, [](int sig){
+    int status;
+    waitpid(-1, &status, WNOHANG);
+  });
+
   child_pid_ = fork();
 
   if (child_pid_ < 0) {