configure.ac: switch to autoconf 2.69

OpenOCD file configure.ac requires autoconf equal or newer then
2.64, which is quite old.
These are the release dates so far:
- autoconf-2.64 2009-07-26
- autoconf-2.65 2009-11-21
- autoconf-2.66 2010-07-02
- autoconf-2.67 2010-08-02
- autoconf-2.68 2010-09-22
- autoconf-2.69 2012-04-24
- autoconf-2.70 2020-12-08

Switch to autoconf 2.69, which is old enough to be the standard in
current systems.
This should reduce the effort on configure.ac compatibility.

Change-Id: Ia7c78d2fa34c50ed5ccf4fb66ad3484369cf3b4a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6008
Tested-by: jenkins

configure.ac

@@ -1,4 +1,4 @@
-AC_PREREQ(2.64)
+AC_PREREQ([2.69])
 AC_INIT([openocd], [0.11.0+dev],
   [OpenOCD Mailing List <openocd-devel@lists.sourceforge.net>])
 AC_CONFIG_SRCDIR([src/openocd.c])