configure syntax error

[Ben Elliston]

When configuring libstdc++, I see the following error among the output:

configure: WARNING: No native atomic operations are provided for this platform.
/home/bje/source/gcc-trunk/libstdc++-v3/configure: line 14808: test: =: unary operator expected
configure: WARNING: They will be faked using a mutex.
configure: WARNING: Performance of certain classes will degrade as a result.

The problem is that this shell code:

      if test $target_thread_file = single; then

.. is invoked when target_thread_file is an unset variable.  This code
is in the macro GLIBCXX_ENABLE_ATOMIC_BUILTINS, which is invoked before
the macros that set target_thread_file in configure.ac:

GLIBCXX_ENABLE_ATOMIC_BUILTINS
GLIBCXX_ENABLE_THREADS

I'm not quite sure how to fix this.  Should these invocations be
reversed?  Should the `if' just be more defensive, like so?

  if test "x$target_thread_file" = xsingle; then

Thanks,
Ben