build: adjust for recent gnulib pthread changes

Discussed in https://lists.gnu.org/r/coreutils/2019-08/msg00030.html .

* bootstrap.conf (gnulib_modules): Replace 'pthread' with
pthread-* modules.
* src/sort.c: Remove GNULIB_defined_pthread_functions conditional.

bootstrap.conf

@@ -196,7 +196,10 @@ gnulib_modules="
   priv-set
   progname
   propername
-  pthread
+  pthread-cond
+  pthread-mutex
+  pthread-thread
+  pthread_sigmask
   putenv
   quote
   quotearg

src/sort.c

@@ -82,11 +82,6 @@ struct rlimit { size_t rlim_cur; };
 # endif
 #endif
 
-#if GNULIB_defined_pthread_functions
-# undef pthread_sigmask
-# define pthread_sigmask(how, set, oset) sigprocmask (how, set, oset)
-#endif
-
 #if !defined OPEN_MAX && defined NR_OPEN
 # define OPEN_MAX NR_OPEN
 #endif