AC_MSG_CHECKING(what kind of backend should we use)
AC_ARG_WITH(pthread, [ --with-pthread Use pthread instead of ucontext.],with_context=pthread)
+AC_ARG_WITH(pthread, [ --with-pthreads Use pthread instead of ucontext.],with_context=pthread)
AC_ARG_WITH(context,
[ --with-context=[ucontext/pthread] Use either (System V) swapcontext or pthread [[default=auto]].],,