PATCH, v3: fix error from configure

[Ian Lance Taylor]

Ben Elliston <bje@au1.ibm.com> writes:

> On some targets, $target_thread_file can be unset (I'm not sure if that
> is the right behaviour or not).  This patch protects against a "unary
> operator expected" error from the configure script.  OK for mainline?

The problem would seem to be that GLIBCXX_ENABLE_THREADS comes after
GLIBCXX_ENABLE_ATOMIC_BUILTINS in libstdc++-v3/configure.ac.  Can you
try just flipping those two?

Ian