Overriding cross_compiling when host != build isn't a good enough test

[Daniel Kegel]

Background:
Currently, autoconf sets cross_compiling=no if host_alias = build_alias.
That's not always correct -- if you want to build from pristine sources,
the best way is to pretend it's a cross-build.  (LFS comes to mind.)
The usual workaround is to use a slightly phony value for host_alias
by modifying the more-or-less unused manufacturer field (e.g. 
i686-HOST-linux-gnu).
This fails for architectures which only have a single manufacturer,
as config.sub will reject phony manufacturer names.
It's also clumsy when doing a canadian cross, as all those phony machine
names get confusing.

My current problem:
I'm doing a canadian cross build of a gcc/glibc toolchain.
gcc is configured with
 --build=i686-pc-linux-gnu --target=x86_64-unknown-linux-gnu 
--host=x86_64-unknown-linux-gnu
and glibc is configured with the machine types shifted by one as usual
(see the famous comment in 
http://gcc.gnu.org/ml/libstdc++/2003-07/msg00461.html,
and another example at 
http://handhelds.org/download/toolchain/gcc-build-cross-3.3):
 --build=x86_64-unknown-linux-gnu --host=x86_64-unknown-linux-gnu
Configuring glibc-2.3.2 fails with the error
  checking size of long double... configure: error: cannot compute sizeof (long 
double), 77
because autoconf-2.57 mistakenly thinks that because build=host, it
can run the output of the C compiler.

Proposed fix:
The simple fix seems to be to allow the user to override cross_compiling
when running configure, e.g.

--- general.m4.old      2004-05-18 12:21:26.000000000 -0700
+++ general.m4  2004-05-18 12:23:18.000000000 -0700
@@ -397,7 +397,7 @@
 #
 ac_default_prefix=/usr/local
 ac_config_libobj_dir=.
-cross_compiling=no
+cross_compiling=${cross_compiling-no}
 subdirs=
 MFLAGS=
 MAKEFLAGS=
@@ -869,7 +869,7 @@
 target=$target_alias

 # FIXME: To remove some day.
-if test "x$host_alias" != x; then
+if test "x$cross_compiling" != "yes" && test "x$host_alias" != x; then
   if test "x$build_alias" = x; then
     cross_compiling=maybe
     AC_MSG_WARN([If you wanted to set the --build type, don't use --host.


What say?