[m-rev.] diff: update README.MacOS for XCode 4 issues

[Julien Fischer]

Branches: main, 11.07

README.MacOS:
 	Explain the situation regarding XCode 4 and llvm-gcc.

Julien.

Index: README.MacOS
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/README.MacOS,v
retrieving revision 1.20
diff -u -r1.20 README.MacOS
--- README.MacOS	25 Aug 2011 17:48:32 -0000	1.20
+++ README.MacOS	23 Nov 2011 05:57:09 -0000
@@ -1,6 +1,21 @@
  This file documents the port of Mercury to Mac OS X,
  i.e. the "*-apple-darwin*" configuration.

+=========================================================
+IMPORTANT NOTE FOR FOR USERS OF MAC OS X 10.7 AND XCODE 4
+=========================================================
+
+The default C compiler provided in XCode 4 is llvm-gcc.
+Mercury does NOT work with llvm-gcc.  In order to build Mercury you will either
+need to use one of the other C compilers available on the system, either GCC
+4.2 or clang.
+
+Alternatively, the MacPorts project, <http://www.macports.org/>, provides more
+recent versions of GCC for Mac OS X.
+
+As llvm-gcc has been deprecated by the LLVM project, we have no intention of
+ever supporting Mercury with it.
+
  ----------------------------------
  Mercury on Mac OS X 10.5 and 10.6
  ----------------------------------
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