build: set AC_PREREQ to 2.69

We use build macros such as AX_CHECK_LINK_FLAG, that require >=2.64, so
our configure should also require Autoconf >= 2.64. The build would
already blow up if 2.64 wasn't available. i.e:

configure.ac:320: error: Autoconf version 2.64 or higher is required
build-aux/m4/ax_check_link_flag.m4:74: AX_CHECK_LINK_FLAG is expanded from...

For reference, Autoconf 2.69 was released in April of 2012.
https://lists.gnu.org/archive/html/autoconf/2012-04/msg00041.html

See https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Versioning.html
for more info on AC_PREREQ.

configure.ac

@@ -1,5 +1,4 @@
-dnl require autoconf 2.60 (AS_ECHO/AS_ECHO_N)
-AC_PREREQ([2.60])
+AC_PREREQ([2.69])
 define(_CLIENT_VERSION_MAJOR, 0)
 define(_CLIENT_VERSION_MINOR, 19)
 define(_CLIENT_VERSION_REVISION, 99)