BPO-41100: Support macOS 11 when building (GH-21113)

(cherry picked from commit 8ea6353f60625c96ce96588c70ff24a77f8c71f9)

Co-authored-by: Ronald Oussoren <ronaldoussoren@mac.com>
diff --git a/configure b/configure
index ca18d2f..9411d7d 100755
--- a/configure
+++ b/configure
@@ -3422,7 +3422,7 @@
   # has no effect, don't bother defining them
   Darwin/[6789].*)
     define_xopen_source=no;;
-  Darwin/1[0-9].*)
+  Darwin/[12][0-9].*)
     define_xopen_source=no;;
   # On AIX 4 and 5.1, mbstate_t is defined only when _XOPEN_SOURCE == 500 but
   # used in wcsnrtombs() and mbsnrtowcs() even if _XOPEN_SOURCE is not defined