Add "fails_with_llvm" to formula to document LLVM build breaks.

Replaced ENV.gcc_4_2 + comments with calls to "fails_with_llvm",
to specifically message to the user when a formula is known or suspected
to not build with LLVM. If the user specifies "--use-llvm", the message
will be displayed, but compilation will be tried anyway.

Since using LLVM is now an advanced/hidden feature instead of the
default on 10.6, we'll let the user try anyway (and submit patches
if things are now working.)

Library/Homebrew/formula.rb

@@ -296,6 +296,17 @@ class Formula
     self.class.external_deps
   end
 
+  def fails_with_llvm msg="", data=nil
+    return unless (ENV['HOMEBREW_USE_LLVM'] or ARGV.include? '--use-llvm')
+
+    build = data.delete :build rescue nil
+    msg = "(No specific reason was given)" if msg.empty?
+
+    opoo "LLVM was requested, but this formula is reported as not working with LLVM:"
+    puts msg
+    puts "Tested with LLVM build #{build}" unless build == nil
+  end
+
 protected
   # Pretty titles the command and buffers stdout/stderr
   # Throws if there's an error