libxml2: Added --HEAD and fixed Python bindings

-  Python 2.7+ is needed for current version. A fix to
   build with Python 2.6 has been recently added to
   the master:
   https://git.gnome.org/browse/libxml2/commit/?id=722923bc9b788f028e4221c77138fc66735a3113

-  So for system python on 10.6, you'll have to use
   the --HEAD for now.

-  Fix audit not detecting the `if python` after
   the def caveats.

Formula/libxml2.rb

@@ -6,11 +6,21 @@ class Libxml2 < Formula
   mirror 'http://xmlsoft.org/sources/libxml2-2.9.1.tar.gz'
   sha256 'fd3c64cb66f2c4ea27e934d275904d92cec494a8e8405613780cbc8a71680fdb'
 
+  head 'https://git.gnome.org/browse/libxml2', :using => :git
+
   keg_only :provided_by_osx
 
   option :universal
 
-  depends_on :python => :recommended
+  if build.head?
+    depends_on :python => :recommended # satisfied by Python 2.6+
+    depends_on :autoconf
+    depends_on :automake
+    depends_on :libtool
+  else
+    # 2.9.1 cannot build with Python 2.6: https://github.com/mxcl/homebrew/issues/20249
+    depends_on PythonInstalled.new("2.7") => :recommended
+  end
 
   fails_with :llvm do
     build 2326
@@ -19,6 +29,10 @@ class Libxml2 < Formula
 
   def install
     ENV.universal_binary if build.universal?
+    if build.head?
+      inreplace 'autogen.sh', 'libtoolize', 'glibtoolize'
+      system './autogen.sh'
+    end
 
     system "./configure", "--disable-dependency-tracking",
                           "--prefix=#{prefix}",
@@ -47,13 +61,15 @@ class Libxml2 < Formula
   end
 
   def caveats
-    <<-EOS.undent
-      Even if this formula is keg_only, the python bindings have been linked
-      into the global site-packages for your convenience.
-        #{python.global_site_packages}
+    if build.with? 'python'
+      <<-EOS.undent
+        Even if this formula is keg_only, the python bindings have been linked
+        into the global site-packages for your convenience.
+          #{python.global_site_packages}
 
       EOS
-  end if build.with? 'python'
+    end
+  end
 
   def test
     if build.with? 'python'