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

diff --git a/configure b/configure
index c51f396..5024860 100755
--- a/configure
+++ b/configure
@@ -3426,7 +3426,7 @@
   # 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