autoconf: _AC_PROG_CC_C99: fix typo (#110396)

https://github.com/libsdl-org/autoconf/commit/05972f49ee632cd98057a3caf82ebfb9574846da

From 05972f49ee632cd98057a3caf82ebfb9574846da Mon Sep 17 00:00:00 2001
From: Zack Weinberg <[EMAIL REDACTED]>
Date: Tue, 8 Dec 2020 21:35:45 -0500
Subject: [PATCH] _AC_PROG_CC_C99: fix typo (#110396)

_AC_PROG_CC_C99 was using the wrong test program.

Fixes #110396, reported anonymously.

* lib/autoconf/c.m4 (_AC_PROG_CC_C99): Use the C99 test program, not
  the C89 test program.
---
 lib/autoconf/c.m4 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index fe217cad..601c2df4 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -1559,7 +1559,7 @@ dnl SVR4			-Xc -D__EXTENSIONS__
 # option to output variable CC to make it so.
 AC_DEFUN([_AC_PROG_CC_C99],
 [AC_REQUIRE([_AC_C_C99_TEST_PROGRAM])]dnl
-[_AC_C_STD_TRY([c99], [ac_c_conftest_c89_program],
+[_AC_C_STD_TRY([c99], [ac_c_conftest_c99_program],
 dnl Try
 dnl GCC		-std=gnu99 (unused restrictive modes: -std=c99 -std=iso9899:1999)
 dnl IBM XL C	-qlanglvl=extc1x (V12.1; does not pass C11 test)