test, which runs afoul of Automake installations where, for example,
/usr/share/aclocal contains a copy of gl_THREADLIB.
Problem reported by Sven Joachim in
<http://lists.gnu.org/archive/html/emacs-devel/2011-07/msg00529.html>.
This is just a quick temporary fix, specific to Emacs; I'll work
with the other gnulib maintainers to get a more-permanent fix.