test unset portably

[Ralf Wildenhues]

Apparently some older bash versions returned 1 upon unsetting an unset
variable.  D'oh!  This should be documented in autoconf.texi (patch to
follow on its list).

Applied to HEAD.  

Cheers,
Ralf

        * tests/testsuite.at: Test `unset' on a set variable; older
        bash versions return 1 if the variable was not set.

Index: tests/testsuite.at
===================================================================
RCS file: /cvsroot/libtool/libtool/tests/testsuite.at,v
retrieving revision 1.43
diff -u -r1.43 testsuite.at
--- tests/testsuite.at  1 Sep 2006 16:19:54 -0000       1.43
+++ tests/testsuite.at  3 Sep 2006 15:18:53 -0000
@@ -35,7 +35,7 @@
 if test -n "$host_alias"; then
   configure_options="--host $host_alias"
 fi
-if (unset FOO) >/dev/null 2>&1; then
+if (FOO=bar; unset FOO) >/dev/null 2>&1; then
   unset=unset
 else
   unset=false