gnu: Add Autoconf 2.64.

* gnu/packages/autotools.scm (autoconf-2.64): New variable.

gnu/packages/autotools.scm

@@ -74,6 +74,20 @@ (define-public autoconf-2.68
        (base32
         "1fjm21k2na07f3vasf288a0zx66lbv0hd3l9bvv3q8p62s3pg569"))))))
 
+(define-public autoconf-2.64
+  ;; As of GDB 7.8, GDB is still developed using this version of Autoconf.
+  (package (inherit autoconf)
+    (version "2.64")
+    (source
+     (origin
+      (method url-fetch)
+      (uri (string-append "mirror://gnu/autoconf/autoconf-"
+                          version ".tar.xz"))
+      (sha256
+       (base32
+        "0j3jdjpf5ly39dlp0bg70h72nzqr059k0x8iqxvaxf106chpgn9j"))))))
+
+
 (define* (autoconf-wrapper #:optional (autoconf autoconf))
   "Return an wrapper around AUTOCONF that generates `configure' scripts that
 use our own Bash instead of /bin/sh in shebangs.  For that reason, it should