Fix execute_process blocking on Windows (#76)

Author: philippewarren

subprocess.hpp

@@ -1330,6 +1330,7 @@ class Popen
 
 #ifdef __USING_WINDOWS__
   HANDLE process_handle_;
+  std::future<void> cleanup_future_;
 #endif
 
   bool defer_process_start_ = false;
@@ -1555,7 +1556,7 @@ inline void Popen::execute_process() noexcept(false)
 
   this->process_handle_ = piProcInfo.hProcess;
 
-  std::async(std::launch::async, [this] {
+  this->cleanup_future_ = std::async(std::launch::async, [this] {
     WaitForSingleObject(this->process_handle_, INFINITE);
 
     CloseHandle(this->stream_.g_hChildStd_ERR_Wr);