Document required autotools versions

CentOS 6 is roughly the oldest thing we care about developers still
using, and it has autoconf 2.63 / automake 1.11.  These are both
older than openssl 1.0.0, so anybody who can't upgrade past those
probably can't upgrade to a modern openssl either.  And since only
people building from git or editing configure.ac/Makefile.am need to
use autotools, I'm not totally enthused about keeping support for
old ones anyway.

Closes ticket 17732.

Makefile.am

@@ -4,8 +4,9 @@
 # See LICENSE for licensing information
 
 # "foreign" means we don't follow GNU package layout standards
-# 1.9 means we require automake vesion 1.9
-AUTOMAKE_OPTIONS = foreign 1.9 subdir-objects
+# 1.11 means we require automake vesion 1.11
+# "subdir-objects" means put .o files in the same directory as the .c files
+AUTOMAKE_OPTIONS = foreign 1.11 subdir-objects
 
 ACLOCAL_AMFLAGS = -I m4
 

changes/bug17732

@@ -0,0 +1,10 @@
+  o Minor features (compilation):
+    - Note our minimum required autoconf/automake versions in the
+      appropriate locations. Closes ticket 17732.
+
+  o Dependency updates:
+    - Tor now uses Autoconf version 2.63 or later, and Automake 1.11 or later
+      (released in 2008 and 2009 respectively).  If you are building Tor from
+      the git repository instead of from the source distribution, and your
+      tools are older than this, you will need to upgrade.
+      Closes ticket 17732.

configure.ac

@@ -3,6 +3,7 @@ dnl Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson
 dnl Copyright (c) 2007-2015, The Tor Project, Inc.
 dnl See LICENSE for licensing information
 
+AC_PREREQ([2.63])
 AC_INIT([tor],[0.2.8.1-alpha-dev])
 AC_CONFIG_SRCDIR([src/or/main.c])
 AC_CONFIG_MACRO_DIR([m4])