Adding joinable threads, and gpr_thd_join.
diff --git a/build.json b/build.json
index 1e7ab96..06adfee 100644
--- a/build.json
+++ b/build.json
@@ -337,6 +337,7 @@
         "src/core/support/sync.c",
         "src/core/support/sync_posix.c",
         "src/core/support/sync_win32.c",
+        "src/core/support/thd.c",
         "src/core/support/thd_posix.c",
         "src/core/support/thd_win32.c",
         "src/core/support/time.c",