CMake: use a requirement to look for expat.framework

Formula/cmake.rb

@@ -1,5 +1,21 @@
 require 'formula'
 
+class NoExpatFramework < Requirement
+  def message; <<-EOS.undent
+    Detected /Library/Frameworks/expat.framework
+
+    This will be picked up by CMake's build system and likely cause the
+    build to fail, trying to link to a 32-bit version of expat.
+
+    You may need to move this file out of the way to compile CMake.
+    EOS
+  end
+  def satisfied?
+    not File.exist? "/Library/Frameworks/expat.framework"
+  end
+end
+
+
 class Cmake < Formula
   url 'http://www.cmake.org/files/v2.8/cmake-2.8.8.tar.gz'
   md5 'ba74b22c788a0c8547976b880cd02b17'
@@ -10,22 +26,14 @@ class Cmake < Formula
     sha1 '4d25a7ca3f41750d957fe1cfd53ecb37c713efad' => :snowleopard
   end
 
-  def patches
+  depends_on NoExpatFramework.new
+
   # Correct FindPkgConfig found variable. Remove for CMake 2.8.9.
+  def patches
     "https://github.com/Kitware/CMake/commit/3ea850.patch"
   end
 
   def install
-    # A framework-installed expat will be detected and mess things up.
-    if File.exist? "/Library/Frameworks/expat.framework"
-      opoo "/Library/Frameworks/expat.framework detected"
-      puts <<-EOS.undent
-        This will be picked up by CMake's build system and likey cause the
-        build to fail, trying to link to a 32-bit version of expat.
-        You may need to move this file out of the way for this brew to work.
-      EOS
-    end
-
     system "./bootstrap", "--prefix=#{prefix}",
                           "--system-libs",
                           "--no-system-libarchive",