Fix keg-only on pre-Mountain Lion formulae.

Formula/cairo.rb

@@ -5,7 +5,7 @@ class Cairo < Formula
   url 'http://cairographics.org/releases/cairo-1.12.2.tar.xz'
   sha256 'b786bc4a70542bcb09f2d9d13e5e6a0c86408cbf6d1edde5f0de807eecf93f96'
 
-  keg_only unless MacOS.version >= :mountain_lion
+  keg_only :provided_pre_mountain_lion
 
   option :universal
   option 'without-x', 'Build without X11 support'

Formula/enscript.rb

@@ -8,7 +8,7 @@ class Enscript < Formula
 
   head 'git://git.savannah.gnu.org/enscript.git'
 
-  keg_only :provided_by_osx unless MacOS.version >= :mountain_lion
+  keg_only :provided_pre_mountain_lion
 
   depends_on 'gettext'
 

Formula/fontconfig.rb

@@ -5,7 +5,7 @@ class Fontconfig < Formula
   url 'http://fontconfig.org/release/fontconfig-2.10.1.tar.gz'
   sha1 'e377cbe989cd22d3a10020309c906ecbbcac0043'
 
-  keg_only unless MacOS.version >= :mountain_lion
+  keg_only :provided_pre_mountain_lion
 
   depends_on :freetype
   depends_on 'pkg-config' => :build

Formula/freetype.rb

@@ -5,7 +5,7 @@ class Freetype < Formula
   url 'http://downloads.sf.net/project/freetype/freetype2/2.4.10/freetype-2.4.10.tar.gz'
   sha1 '44dba26ff965b1cd1c62e480fdefaeca62ed33da'
 
-  keg_only unless MacOS.version >= :mountain_lion
+  keg_only :provided_pre_mountain_lion
 
   option :universal
 

Formula/libpng.rb

@@ -5,7 +5,7 @@ class Libpng < Formula
   url 'http://downloads.sf.net/project/libpng/libpng15/1.5.12/libpng-1.5.12.tar.gz'
   sha1 'c329f3a9b720d7ae14e8205fa6e332236573704b'
 
-  keg_only unless MacOS.version >= :mountain_lion
+  keg_only :provided_pre_mountain_lion
 
   bottle do
     sha1 '83c6be83e86404f41982e5e1e6877924fe737bdf' => :mountainlion

Formula/pixman.rb

@@ -7,7 +7,7 @@ class Pixman < Formula
 
   depends_on 'pkg-config' => :build
 
-  keg_only unless MacOS.version >= :mountain_lion
+  keg_only :provided_pre_mountain_lion
 
   option :universal