Add references for Xcode and the Python Developer's Guide

to the internal Mac/README file.

Mac/README

@@ -81,6 +81,13 @@ you can use the system headers instead of an SDK::
 
   $ ./configure --enable-universalsdk=/
 
+In general, universal builds depend on specific features provided by the
+Apple-supplied compilers and other build tools included in Apple's Xcode
+development tools.  You should install Xcode and the command line tools
+component appropriate for the OS X release you are running on.  See the
+Python Developer's Guide (http://docs.python.org/devguide/setup.html)
+for more information.
+
 2.1 Flavours of universal binaries
 ..................................