From 24fa9b2a04ff24d0f10aa3882e4069c561a2ef04 Mon Sep 17 00:00:00 2001
From: Alexey Kopytov <akopytov@gmail.com>
Date: Mon, 13 Feb 2017 21:52:41 +0300
Subject: [PATCH] Do not require Autoconf 2.64, as CentOS 6 provides 2.63.

The only required bit from Autoconf 2.64 is AS_VAR_IF. Which is in fact
provided, but just in case there is now a fallback implementation in
m4/sb_autoconf_compat.m4.
---
 m4/ax_check_compile_flag.m4 |  2 +-
 m4/sb_autoconf_compat.m4    | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)
 create mode 100644 m4/sb_autoconf_compat.m4

diff --git a/m4/ax_check_compile_flag.m4 b/m4/ax_check_compile_flag.m4
index ca36397..99a2220 100644
--- a/m4/ax_check_compile_flag.m4
+++ b/m4/ax_check_compile_flag.m4
@@ -58,7 +58,7 @@
 #serial 4
 
 AC_DEFUN([AX_CHECK_COMPILE_FLAG],
-[AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF
+[
 AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
 AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
   ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
diff --git a/m4/sb_autoconf_compat.m4 b/m4/sb_autoconf_compat.m4
new file mode 100644
index 0000000..00c4770
--- /dev/null
+++ b/m4/sb_autoconf_compat.m4
@@ -0,0 +1,13 @@
+# ---------------------------------------------------------------------------
+# Provide various compatibility macros for older Autoconf machines
+# Definitions were copied from the Autoconf source code.
+# ---------------------------------------------------------------------------
+m4_ifdef([AS_VAR_IF],,m4_define([AS_VAR_IF],
+[AS_LITERAL_WORD_IF([$1],
+  [AS_IF(m4_ifval([$2], [[test "x$$1" = x[]$2]], [[${$1:+false} :]])],
+  [AS_VAR_COPY([as_val], [$1])
+   AS_IF(m4_ifval([$2], [[test "x$as_val" = x[]$2]], [[${as_val:+false} :]])],
+  [AS_IF(m4_ifval([$2],
+    [[eval test \"x\$"$1"\" = x"_AS_ESCAPE([$2], [`], [\"$])"]],
+    [[eval \${$1:+false} :]])]),
+[$3], [$4])]))dnl