https://git.ruby-lang.org/ruby.git/commit/?id=8b32de2ec9

From 8b32de2ec9b72d4c9ede19b70ec9497718fb25a6 Mon Sep 17 00:00:00 2001
From: Benoit Daloze <eregontp@g...>
Date: Thu, 29 Apr 2021 14:14:52 +0200
Subject: Add -Werror=undef to default warnflags for core

* See [Feature #17752]
* For external extensions it's transformed to just warn and not error (-Wundef)
  like other other -Werror in warnflags.
---
 configure.ac | 1 +
 1 file changed, 1 insertion(+)

diff --git a/configure.ac b/configure.ac
index fa62b5f..6605939 100644
--- a/configure.ac
+++ b/configure.ac
@@ -627,6 +627,7 @@ AS_CASE(["$GCC:${warnflags+set}:${extra_warnflags:+set}:"], https://github.com/ruby/ruby/blob/trunk/configure.ac#L627
 		 -Werror=pointer-arith \
 		 -Werror=shorten-64-to-32 \
 		 -Werror=write-strings \
+		 -Werror=undef \
 		 -Wimplicit-fallthrough=0 \
 		 -Wmissing-noreturn \
 		 -Wno-cast-function-type \
-- 
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