https://github.com/libsdl-org/autoconf/commit/b27bc3e230bb12fdd9a813e38e82bc4c3e22b4cc
From b27bc3e230bb12fdd9a813e38e82bc4c3e22b4cc Mon Sep 17 00:00:00 2001
From: Paul Eggert <[EMAIL REDACTED]>
Date: Tue, 31 Aug 2021 16:30:46 -0700
Subject: [PATCH] Port AC_LANG_CALL(C) to C++
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* lib/autoconf/c.m4 (AC_LANG_CALL(C)): Add an extern "C" if C++.
Problem reported by Vincent Lefèvre (sr #110532).
---
lib/autoconf/c.m4 | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index 44443a39..48bd49a3 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -126,7 +126,13 @@ m4_define([AC_LANG_CALL(C)],
m4_if([$2], [main], ,
[/* Override any GCC internal prototype to avoid an error.
Use char because int might match the return type of a GCC
- builtin and then its argument prototype would still apply. */
+ builtin and then its argument prototype would still apply.
+ The 'extern "C"' is for builds by C++ compilers;
+ although this is not generally supported in C code, supporting it here
+ has little cost and some practical benefit (sr 110532). */
+#ifdef __cplusplus
+extern "C"
+#endif
char $2 ();])], [return $2 ();])])