[3.8] bpo-39794: Add --without-decimal-contextvar (GH-18702)

(cherry picked from commit 815280eb160af637e1347213659f9236adf78f80)
diff --git a/PC/pyconfig.h b/PC/pyconfig.h
index b40e24f..b6b8d44 100644
--- a/PC/pyconfig.h
+++ b/PC/pyconfig.h
@@ -470,6 +470,10 @@
    (which you can't on SCO ODT 3.0). */
 /* #undef SYS_SELECT_WITH_SYS_TIME */
 
+/* Define if you want build the _decimal module using a coroutine-local rather
+   than a thread-local context */
+#define WITH_DECIMAL_CONTEXTVAR 1
+
 /* Define if you want documentation strings in extension modules */
 #define WITH_DOC_STRINGS 1