support cross multilibs

[Geoffrey Keating]

Sometimes, a target has a multilib which doesn't actually work on
host for that target.

The particular example I'm thinking of is that on i386-darwin, there
is a -m64 multilib for 64-bit programs, and all darwin9 hosts support
building using this multilib, but not all darwin hosts are
capable of running 64-bit programs.  A similar situation occurs on
powerpc-darwin.  On some other systems, there are multilibs that make
the ABI different (like -msoft-float); sometimes these are 'close
enough' that programs can be run, sometimes not.

The problem with this is that a native build, using 'configure &&
make' with no special options, is not a cross compilation
and so autoconf will check that built programs can be run, and if they
don't run configure will error.  This is fine for the default
multilib, since that ought to work, but not fine for -m64 on a 32-bit
processor.

So, I'd like to put in this patch, which adds a little code to
AM_MULTILIB to make non-default multilibs possibly be considered to be
cross compilation.  autoconf will check whether compiling actually
fails.

I tested this by building development GCC 4.2 on a Darwin machine with
an Intel Core Duo (not to be confused with Core 2 Duo) with
'$srcdir/configure && make' after regenerating the libstdc++-v3,
libssp, libgomp, and libobjc copies of aclocal.m4 and configure with a
automake 1.9.6 with this patch applied.  (multi.m4 is identical in
1.9.6 and mainline.)

I'd appreciate it if this could go on the 1.9.x branch as well as
mainline.

-- 
- Geoffrey Keating <address@hidden>

===File ~/patches/automake-multilibcross.patch==============
2006-09-14  Geoffrey Keating  <address@hidden>

        * m4/multi.m4: Non-default multilibs may be cross compilation.

Index: m4/multi.m4
===================================================================
RCS file: /cvs/automake/automake/m4/multi.m4,v
retrieving revision 1.14
diff -u -p -r1.14 multi.m4
--- m4/multi.m4 9 Jan 2005 14:46:21 -0000       1.14
+++ m4/multi.m4 14 Sep 2006 23:54:10 -0000
@@ -1,12 +1,12 @@
 ##                                                          -*- Autoconf -*-
-# Copyright (C) 1998, 1999, 2000, 2001, 2003, 2004, 2005
+# Copyright (C) 1998, 1999, 2000, 2001, 2003, 2004, 2005, 2006
 # Free Software Foundation, Inc.
 #
 # This file is free software; the Free Software Foundation
 # gives unlimited permission to copy and/or distribute it,
 # with or without modifications, as long as this notice is preserved.
 
-# serial 5
+# serial 6
 
 # AM_ENABLE_MULTILIB([MAKEFILE], [REL-TO-TOP-SRCDIR])
 # ---------------------------------------------------
@@ -37,6 +37,13 @@ else
 fi
 AC_SUBST(multi_basedir)
 
+# Even if the default multilib is not a cross compilation,
+# it may be that some of the other multilibs are.
+if test $cross_compiling = no && test $multilib = yes \
+   && test "x${with_multisubdir}" != x ; then
+   cross_compiling=maybe
+fi
+
 AC_OUTPUT_COMMANDS([
 # Only add multilib support code if we just rebuilt the top-level
 # Makefile.
============================================================