Add text to Mac/README to warn about non-universal libraries when

building a universal Python. Based on issue7679.

Mac/README

@@ -224,6 +224,25 @@ Because of the way the script locates the files it needs you have to run it
 from within the BuildScript directory. The script accepts a number of 
 command-line arguments, run it with --help for more information.
 
+Configure warnings
+==================
+
+The configure script sometimes emits warnings like the one below::
+
+   configure: WARNING: libintl.h: present but cannot be compiled
+   configure: WARNING: libintl.h:     check for missing prerequisite headers?
+   configure: WARNING: libintl.h: see the Autoconf documentation
+   configure: WARNING: libintl.h:     section "Present But Cannot Be Compiled"
+   configure: WARNING: libintl.h: proceeding with the preprocessor's result
+   configure: WARNING: libintl.h: in the future, the compiler will take precedence
+   configure: WARNING:     ## -------------------------------------- ##
+   configure: WARNING:     ## Report this to http://bugs.python.org/ ##
+   configure: WARNING:     ## -------------------------------------- ##
+
+This almost always means you are trying to build a universal binary for
+Python and have libaries in ``/usr/local`` that don't contain the required
+architectures. Temporarily move ``/usr/local`` aside to finish the build.
+
 Odds and ends
 =============