[PATCH][gdb/build] Fix build with undefined CXX_STD_THREAD

[Tom de Vries via Gdb-patches <gdb-patches@sourceware.org>]

Hi,

When building gdb on openSUSE Leap 42.3, we trigger the case that
CXX_STD_THREAD is undefined, and run into:
...
gdb/maint.c: In function ‘void maintenance_show_worker_threads \
  (ui_file*, int, cmd_list_element*, const char*)’:
gdb/maint.c:877:14: error: ‘gdb::thread_pool’ has not been declared
         gdb::thread_pool::g_thread_pool->thread_count ());
              ^
Makefile:1647: recipe for target 'maint.o' failed
make[1]: *** [maint.o] Error 1
...

Fix this by handling the undefined CXX_STD_THREAD case in
maintenance_show_worker_threads, such that we get:
...
$ gdb -q -batch -ex "maint show worker-thread"
The number of worker threads GDB can use is 0.
...

Tested on x86_64-linux.

Any comments?

Thanks,
- Tom

[gdb/build] Fix build with undefined CXX_STD_THREAD

---
 gdb/maint.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/gdb/maint.c b/gdb/maint.c
index cbd7d4ed47b..56a197c2a14 100644
--- a/gdb/maint.c
+++ b/gdb/maint.c
@@ -871,6 +871,7 @@ maintenance_show_worker_threads (struct ui_file *file, int from_tty,
 				 struct cmd_list_element *c,
 				 const char *value)
 {
+#if CXX_STD_THREAD
   if (n_worker_threads == -1)
     fprintf_filtered (file, _("The number of worker threads GDB "
 			      "can use is unlimited (currently %zu).\n"),
@@ -879,6 +880,10 @@ maintenance_show_worker_threads (struct ui_file *file, int from_tty,
     fprintf_filtered (file, _("The number of worker threads GDB "
 			      "can use is %d.\n"),
 		      n_worker_threads);
+#else
+  fprintf_filtered (file, _("The number of worker threads GDB "
+			    "can use is 0.\n"));
+#endif
 }
 
 \f