libcdecl: Actually test threading support.

Nothing actually runs libcdecl's error messaging in multiple threads to
check if it actually works.  Let's fix that with a new test application.

It does mostly work, but it turns out that we leak memory on Windows
because gnulib doesn't actually implement TLS destructors in a useful
way (on the other hand, using --enable-threads=posix should work even
on Windows with a suitable Windows pthreads library implementation).