[PATCH] fix bug in autoconf-2.13 that keeps cross gcc from buildingon cygwin

[Dan Kegel <dkegel at ixiacom dot com>]

Building cross gcc's on cygwin fails because autoconf 2.13's AC_TRY_COMPILER
test assumes that it's ok to try to run possibly cross-compiled binaries,
and that if they run, the compiler must not be a cross-compiler.
This assumption fails on Cygwin; see
 http://www.oarcorp.com/rtems/maillistArchives/rtems-users/1999/may/msg00075.html
 http://gcc.gnu.org/ml/gcc-help/2002-05/msg00165.html
 http://sources.redhat.com/ml/crossgcc/2002-08/msg00099.html
The symptom is the build hangs about twenty times waiting for you to click a dialog box,
and the target directory's ac_cv_prog_cc_cross is improperly set to 'no'.

The problem will likely go away when gcc moves to using autoconf 2.5x, but
that may take a while, and won't help people who need to build oldish
gcc's like 3.0.x.  So it's worth fixing autoconf-2.13 if it's a small, safe fix.

Fortunately, the fix does look small and safe:

--- /usr/share/autoconf2.13/acgeneral.m4.orig	Thu Aug 22 18:26:58 2002
+++ acgeneral.m4	Thu Aug 22 19:03:12 2002
@@ -1510,11 +1510,13 @@
 EOF
 if AC_TRY_EVAL(ac_link) && test -s conftest${ac_exeext}; then
   [$2]=yes
-  # If we can't run a trivial program, we are probably using a cross compiler.
-  if (./conftest; exit) 2>/dev/null; then
-    [$3]=no
-  else
-    [$3]=yes
+  if test "$[$3]" != yes; then
+    # If we can't run a trivial program, we are probably using a cross compiler.
+    if (./conftest; exit) 2>/dev/null; then
+      [$3]=no
+    else
+      [$3]=yes
+    fi
   fi
 else
   echo "configure: failed program was:" >&AC_FD_CC

i.e. after the patch, AC_TRY_COMPILER checks its 3rd parameter,
and if it's already 'yes', it knows not to run the binaries.

After applying this patch to autoconf-2.13 and installing,
you then need to regenerate libiberty/configure,
gcc/configure and fastjar/configure.  It might be good if future releases
of gcc that still used autoconf-2.13 were done on machines with an
autoconf-2.13 with this patch applied.

This is only a partial fix; it requires the user to override
ac_cv_prog_cc_cross.  A similar bug in ltconfig can be worked around
without a patch by overriding cross_compiling=yes in the same way.
Both overrides should only be done during the build of runtime libraries.
Fortunately, gcc's makefile has pseudotargets that let you do this.
For instance:
    make all-gcc
    ac_cv_prog_cc_cross=yes cross_compiling=yes make all-target
    make install

Users on non-cygwin platforms can ignore this issue, and just do
'make install' as usual.  This patch will neither help nor hurt them.

(Note that overriding ac_cv_prog_cc_cross at make time appears to be sufficient.
Although it would be more logical to override them when doing initial
configuration of gcc, that wouldn't let you work around the ltconfig bug.)

Even if future autoconf and gcc releases don't apply this patch, maybe
this post will help people who need to build gcc on cygwin.

- Dan

http://www.kegel.com