autoconf: doc: fix typo in variable name

https://github.com/libsdl-org/autoconf/commit/f982d9e4e14d84df4801a4354d508b2c66e6166e

From f982d9e4e14d84df4801a4354d508b2c66e6166e Mon Sep 17 00:00:00 2001
From: Emanuele Giaquinta <[EMAIL REDACTED]>
Date: Fri, 14 May 2021 09:07:23 +0300
Subject: [PATCH] doc: fix typo in variable name

Message-Id: <YJ4Tm3WEkv86L/YV@FSAPPLE2215.fi.f-secure.com>
(tiny change)
---
 doc/autoconf.texi | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 7b42dd6a..33cebc37 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -7887,7 +7887,7 @@ ISO Standard C++ features with extensions, preferring the newest edition
 of the C++ standard that is supported.  Currently the newest edition
 Autoconf knows how to detect support for is ISO C++ 2011.  After calling
 this macro, you can check whether the C++ compiler has been set to
-accept standard C++ by inspecting the shell variable @code{ac_prog_cc_stdc}.
+accept standard C++ by inspecting the shell variable @code{ac_prog_cxx_stdcxx}.
 Its value will be @samp{cxx11} or @samp{cxx98}, respectively,
 if the C++ compiler has been set to use the 2011 or 1990 edition of the
 C++ standard, and @samp{no} if the compiler does not support compiling