autoconf: Port AC_C_BIGENDIAN to recent clang

https://github.com/libsdl-org/autoconf/commit/d04cfbfcd861a2842adaaf767d056a4d8e2de4a9

From d04cfbfcd861a2842adaaf767d056a4d8e2de4a9 Mon Sep 17 00:00:00 2001
From: Paul Eggert <[EMAIL REDACTED]>
Date: Wed, 29 May 2019 13:00:26 -0700
Subject: [PATCH] Port AC_C_BIGENDIAN to recent clang

Problem and trivial patch reported by Matthieu Gautier in:
https://lists.gnu.org/r/bug-autoconf/2019-05/msg00006.html
* lib/autoconf/c.m4 (AC_C_BIGENDIAN): Use unsigned short for
values greater than 2**15 - 1.
(cherry picked from commit 4677fc349ce759069c9dc8f099a72e77651f1f7b)
---
 lib/autoconf/c.m4 | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index a4fa5d66..70919ddf 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -1553,16 +1553,16 @@ AC_DEFUN([AC_C_BIGENDIAN],
 	[# Try to guess by grepping values from an object file.
 	 AC_COMPILE_IFELSE(
 	   [AC_LANG_PROGRAM(
-	      [[short int ascii_mm[] =
+	      [[unsigned short int ascii_mm[] =
 		  { 0x4249, 0x4765, 0x6E44, 0x6961, 0x6E53, 0x7953, 0 };
-		short int ascii_ii[] =
+		unsigned short int ascii_ii[] =
 		  { 0x694C, 0x5454, 0x656C, 0x6E45, 0x6944, 0x6E61, 0 };
 		int use_ascii (int i) {
 		  return ascii_mm[i] + ascii_ii[i];
 		}
-		short int ebcdic_ii[] =
+		unsigned short int ebcdic_ii[] =
 		  { 0x89D3, 0xE3E3, 0x8593, 0x95C5, 0x89C4, 0x9581, 0 };
-		short int ebcdic_mm[] =
+		unsigned short int ebcdic_mm[] =
 		  { 0xC2C9, 0xC785, 0x95C4, 0x8981, 0x95E2, 0xA8E2, 0 };
 		int use_ebcdic (int i) {
 		  return ebcdic_mm[i] + ebcdic_ii[i];