Include ARCHFLAGS warning for OS X

ext/extconf.rb

@@ -1,7 +1,39 @@
 #!/usr/bin/env ruby
 require 'mkmf'
 
-dir_config('db')
+inc, lib = dir_config('db')
+
+# OS X compatibility
+if(PLATFORM =~ /darwin/) then
+	# test if Bdb is probably universal
+	
+	filetype = (IO.popen("file #{inc}/../db_dump").readline.chomp rescue nil)
+	# if it's not universal, ARCHFLAGS should be set
+	if((filetype !~ /universal binary/) && ENV['ARCHFLAGS'].nil?) then
+		arch = (IO.popen("uname -m").readline.chomp rescue nil)
+		$stderr.write %{
+===========   WARNING   ===========
+
+You are building this extension on OS X without setting the 
+ARCHFLAGS environment variable, and BerkeleyDB does not appear 
+to have been built as a universal binary. If you are seeing this 
+message, that means that the build will probably fail.
+
+Try setting the environment variable ARCHFLAGS 
+to '-arch #{arch}' before building.
+
+For example:
+(in bash) $ export ARCHFLAGS='-arch #{arch}'
+(in tcsh) % setenv ARCHFLAGS '-arch #{arch}'
+
+Then try building again.
+
+===================================
+
+}
+		# We don't exit here. Who knows? It might build.
+	end
+end
 
 versions=%w(db-4.7 db-4.6 db-4.5 db-4.4 db-4.3 db-4.2)
 until versions.empty?