title: Ruby-build giving me Autoconf version 2.67 or higher is required
body: <p>Trying to install Ruby 2.2.0-dev using ruby-build gives me an error about Autoconf:</p>
  
  <pre><code>$ rbenv install 2.2.0-dev
  Cloning https://github.com/ruby/ruby.git...
  Installing ruby-2.2.0-dev...
  
  BUILD FAILED
  
  Inspect or clean up the working tree at /var/folders/3x/y_8y8vr53ws_kxj97km79q5h0000gn/T/ruby-build.20140107115843.2294
  Results logged to /var/folders/3x/y_8y8vr53ws_kxj97km79q5h0000gn/T/ruby-build.20140107115843.2294.log
  
  Last 10 log lines:
  /var/folders/3x/y_8y8vr53ws_kxj97km79q5h0000gn/T/ruby-build.20140107115843.2294 ~/bogus/path
  Cloning into 'ruby-2.2.0-dev'...
  /var/folders/3x/y_8y8vr53ws_kxj97km79q5h0000gn/T/ruby-build.20140107115843.2294/ruby-2.2.0-dev /var/folders/3x/y_8y8vr53ws_kxj97km79q5h0000gn/T/ruby-build.20140107115843.2294 ~/bogus/path
  configure.in:6: error: Autoconf version 2.67 or higher is required
  configure.in:6: the top level
  autom4te: /usr/bin/gm4 failed with exit status: 63
  </code></pre>
  
  <p>Homebrew seems to claim that autoconf 2.69 is available. Unless it's only using autoconf for itself, without allowing it to be used by others:</p>
  
  <pre><code>$ brew list autoconf
  /usr/local/Cellar/autoconf/2.69/bin/ifnames
  /usr/local/Cellar/autoconf/2.69/bin/autoupdate
  /usr/local/Cellar/autoconf/2.69/bin/autoscan
  /usr/local/Cellar/autoconf/2.69/bin/autoreconf
  /usr/local/Cellar/autoconf/2.69/bin/autom4te
  /usr/local/Cellar/autoconf/2.69/bin/autoheader
  /usr/local/Cellar/autoconf/2.69/bin/autoconf
  /usr/local/Cellar/autoconf/2.69/share/autoconf/ (42 files)
  /usr/local/Cellar/autoconf/2.69/share/emacs/ (4 files)
  /usr/local/Cellar/autoconf/2.69/share/man/ (9 files)
  </code></pre>
  
  <p>I don't think homebrew is using autoconf for itself (keg-only?), because doing <code>brew upgrade autoconf</code> gave me <code>Error: autoconf-2.69 already installed</code>.</p>
  
  <p>However, when I run autoconf, it claims that it's only version 2.61:</p>
  
  <pre><code>$ autoconf --version
  autoconf (GNU Autoconf) 2.61
  </code></pre>
  
  <p>My <code>$PATH</code>:</p>
  
  <pre><code>/Users/agrimm/.rbenv/shims:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin:/usr/local/git/bin
  </code></pre>
  
  <p>My uname:</p>
  
  <pre><code>$ uname -a
  Darwin [redacted] 11.4.2 Darwin Kernel Version 11.4.2: Thu Aug 23 16:25:48 PDT 2012; root:xnu-1699.32.7~1/RELEASE_X86_64 x86_64
  </code></pre>
  
  <p>How do I fix the error in installing 2.2.0-dev?</p>
  
  <p>Does ruby-build use its own autoconf, or does it only use whatever is available to the user? If so, what is the recommended way to make autoconf 2.67 or higher available to ruby-build?</p>