[m-rev.] for review: add a note to README.MacOS

[Julien Fischer]

Branches: main, 10.04

README.MacOS:
 	Add about a note about a problem that can arise when bootstrapping a 64-bit
 	Mercury compiler using a 32-bit one and vice versa.

Julien.

Index: README.MacOS
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/README.MacOS,v
retrieving revision 1.13
diff -u -r1.13 README.MacOS
--- README.MacOS	20 May 2010 05:09:27 -0000	1.13
+++ README.MacOS	9 Jun 2010 03:06:27 -0000
@@ -25,6 +25,21 @@
  Mac OS X 10.5.  To build and install 64-bit Mercury on that system you must
  pass the "-m64" option to gcc.

+Note that if you are building Mercury from scratch using an existing installed
+Mercury compiler, i.e. you are not installing using the pre-compiled C files in
+the source distribution, and you are using a 32-bit Mercury compiler to
+bootstrap a 64-bit installation (or vice versa) then you must compile with
+--cross-compiling enabled.  Not enabling it will result in library installation
+aborting with an error message like the following:
+
+    Uncaught Mercury exception:
+    Software Error: pred_table.m: Unexpected: can't locate compare/3
+
+You can enable cross-compilation by putting the following line in a file named
+Mmake.params at the top-level of the source tree:
+
+    MCFLAGS = --cross-compiling
+
  Mercury can currently only be compiled by gcc on Mac OS X.  It cannot currently
  be compiled with llvm-gcc or clang.


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