title: brew install libxml2 failed using osX 10.8.4
body: <p>Here is my home brew config</p>
  
  <pre><code>~  brew update
  Already up-to-date.
   ~  brew doctor
  Your system is ready to brew.
  ~  brew --config                      
  HOMEBREW_VERSION: 0.9.4
  ORIGIN: https://github.com/mxcl/homebrew.git
  HEAD: 8efa77e7c52200d5f457ba340f71c119bd6cd528
  HOMEBREW_PREFIX: /usr/local
  HOMEBREW_CELLAR: /usr/local/Cellar
  CPU: quad-core 64-bit ivybridge
  OS X: 10.8.4-x86_64
  Xcode: 4.6.3
  CLT: 4.6.0.0.1.1365549073
  LLVM-GCC: build 2336
  Clang: 4.2 build 425
  X11: 2.7.4 =&gt; /opt/X11
  System Ruby: 1.8.7-358
  Perl: /usr/bin/perl
  Python: /usr/local/bin/python =&gt; /Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7
  Ruby: /Users/bill/.rvm/bin/ruby
  </code></pre>
  
  <p>Here is the out put for brew install libxml2.</p>
  
  <pre><code>~  brew install libxml2              
  ==&gt; Downloading ftp://xmlsoft.org/libxml2/libxml2-2.9.1.tar.gz
  Already downloaded: /Library/Caches/Homebrew/libxml2-2.9.1.tar.gz
  ==&gt; ./configure --prefix=/usr/local/Cellar/libxml2/2.9.1 --without-python
  ==&gt; make
  ==&gt; make install
  ==&gt; /usr/local/bin/python setup.py install --prefix=/usr/local/Cellar/libxml2/2.9.1
  cc -fno-strict-aliasing -fno-common -dynamic -arch ppc -arch i386 -g -O2 -DNDEBUG -g -O3 -D_REENTRANT=1 -I/usr/local/Cellar/libxml2/2.9.1/include/libxml2 -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk/usr/include -I/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c libxml2-py.c -o build/temp.macosx-10.3-fat-2.7/libxml2-py.o
  clang: warning: not using the clang compiler for the 'powerpc' architecture
  llvm-gcc-4.2: error trying to exec '/usr/bin/../llvm-gcc-4.2/bin/powerpc-apple-darwin11-llvm-gcc-4.2': execvp: No such file or directory
  clang: error: gcc frontend command failed with exit code 255 (use -v to see invocation)
  error: command 'cc' failed with exit status 255
  
  READ THIS: https://github.com/mxcl/homebrew/wiki/troubleshooting
  
  These open issues may also help:
      https://github.com/mxcl/homebrew/issues/20265
  </code></pre>
  
  <p>I have made sure that my xCode is up to date (Version 4.6.3) and have installed the command line tools. Permissions on /usr/local and /usr/local/Cellar are set properly. What am I missing?</p>