Merge #17769: build: set AC_PREREQ to 2.69

4f4ae6f97e build: set AC_PREREQ to 2.69 (fanquake)

Pull request description:

  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:
  ```bash
  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 the [Autoconf Versioning docs](https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Versioning.html) for more info on `AC_PREREQ`.

ACKs for top commit:
  hebasto:
    re-ACK 4f4ae6f97e, Autoconf 2.69 seems wide available.
  laanwj:
    ACK 4f4ae6f97e

Tree-SHA512: b77de9164ae6667513d40edaf9e16c6e7734c100643297b2dbb2ff54072774fdeab7b3b15d52979b99e204c1c4dcca4725ff155d7f6fdab7a867629130e10185

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)