Allow thread-pool.h to work without threads

thread-pool.h requires CXX_STD_THREAD in order to even be included.
However, there's no deep reason for this, and during review we found
that one patch in the new DWARF indexer series unconditionally
requires the thread pool.

Because the thread pool already allows a task to be run in the calling
thread (for example if it is configured to have no threads in the
pool), it seemed straightforward to make this code ok to use when host
threads aren't available at all.

This patch implements this idea.  I built it on a thread-less host
(mingw, before my recent configure patch) and verified that the result
builds.

After the thread-pool change, parallel-for.h no longer needs any
CXX_STD_THREAD checks at all, so this patch removes these as well.

1 files changed, 9 insertions, 1 deletions

diff --git a/gdbsupport/thread-pool.h b/gdbsupport/thread-pool.h
index 68210664525..2672e4d739a 100644
--- a/gdbsupport/thread-pool.h
+++ b/gdbsupport/thread-pool.h
@@ -21,11 +21,13 @@
 #define GDBSUPPORT_THREAD_POOL_H
 
 #include <queue>
-#include <thread>
 #include <vector>
 #include <functional>
+#if CXX_STD_THREAD
+#include <thread>
 #include <mutex>
 #include <condition_variable>
+#endif
 #include <future>
 #include "gdbsupport/gdb_optional.h"
 
@@ -53,7 +55,11 @@ public:
   /* Return the number of executing threads.  */
   size_t thread_count () const
   {
+#if CXX_STD_THREAD
     return m_thread_count;
+#else
+    return 0;
+#endif
   }
 
   /* Post a task to the thread pool.  A future is returned, which can
@@ -64,6 +70,7 @@ private:
 
   thread_pool () = default;
 
+#if CXX_STD_THREAD
   /* The callback for each worker thread.  */
   void thread_function ();
 
@@ -83,6 +90,7 @@ private:
      between the main thread and the worker threads.  */
   std::condition_variable m_tasks_cv;
   std::mutex m_tasks_mutex;
+#endif /* CXX_STD_THREAD */
 };
 
 }