Fix building 'usr.bin/sort' with threads support

[zakharov.vv_gmail.com]

Summary

When WITH_SORT_THREADS is set, usr.bin/sort build fails with this errors:

/usr/src/usr.bin/sort/radixsort.c:144:17: error: mutex 'g_ls_mutex' is not held on every path through here [-Werror,-Wthread-safety-analysis]
        new_ls->next = g_ls;
                       ^
/usr/src/usr.bin/sort/radixsort.c:141:3: note: mutex acquired here
                pthread_mutex_lock(&g_ls_mutex);
                ^
/usr/src/usr.bin/sort/radixsort.c:149:3: error: releasing mutex 'g_ls_mutex' that was not held [-Werror,-Wthread-safety-analysis]
                pthread_mutex_unlock(&g_ls_mutex);
                ^
2 errors generated.
*** Error code 1

It seems to be ThreadSafetyAnasysis false positive (http://clang.llvm.org/docs/ThreadSafetyAnalysis.html#conditional-locks).

Patch fixes build by setting NO_WERROR when WITH_SORT_THREADS is set.