Fix new pthread code to respect --disable-thread-safety.

Don't try to compile src/port/pthread_barrier_wait.c if we opted out of
threads at configure time.  Revealed by build farm member gaur, which
can't compile this code because of problems with its pthread
implementation.  It shouldn't be trying to, because it's using
--disable-thread-safety.

Defect in commit 44bf3d50.
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2568537.1615603606%40sss.pgh.pa.us

configure

@@ -11635,62 +11635,6 @@ if test "$ac_res" != no; then :
 
 fi
 
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for library containing pthread_barrier_wait" >&5
-$as_echo_n "checking for library containing pthread_barrier_wait... " >&6; }
-if ${ac_cv_search_pthread_barrier_wait+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-  ac_func_search_save_LIBS=$LIBS
-cat confdefs.h - <<_ACEOF >conftest.$ac_ext
-/* end confdefs.h.  */
-
-/* Override any GCC internal prototype to avoid an error.
-   Use char because int might match the return type of a GCC
-   builtin and then its argument prototype would still apply.  */
-#ifdef __cplusplus
-extern "C"
-#endif
-char pthread_barrier_wait ();
-int
-main ()
-{
-return pthread_barrier_wait ();
-  ;
-  return 0;
-}
-_ACEOF
-for ac_lib in '' pthread; do
-  if test -z "$ac_lib"; then
-    ac_res="none required"
-  else
-    ac_res=-l$ac_lib
-    LIBS="-l$ac_lib  $ac_func_search_save_LIBS"
-  fi
-  if ac_fn_c_try_link "$LINENO"; then :
-  ac_cv_search_pthread_barrier_wait=$ac_res
-fi
-rm -f core conftest.err conftest.$ac_objext \
-    conftest$ac_exeext
-  if ${ac_cv_search_pthread_barrier_wait+:} false; then :
-  break
-fi
-done
-if ${ac_cv_search_pthread_barrier_wait+:} false; then :
-
-else
-  ac_cv_search_pthread_barrier_wait=no
-fi
-rm conftest.$ac_ext
-LIBS=$ac_func_search_save_LIBS
-fi
-{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_search_pthread_barrier_wait" >&5
-$as_echo "$ac_cv_search_pthread_barrier_wait" >&6; }
-ac_res=$ac_cv_search_pthread_barrier_wait
-if test "$ac_res" != no; then :
-  test "$ac_res" = "none required" || LIBS="$ac_res $LIBS"
-
-fi
-
 # Solaris:
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for library containing fdatasync" >&5
 $as_echo_n "checking for library containing fdatasync... " >&6; }
@@ -11978,6 +11922,65 @@ if test "$ac_res" != no; then :
 fi
 
 
+if test "$enable_thread_safety" = yes; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for library containing pthread_barrier_wait" >&5
+$as_echo_n "checking for library containing pthread_barrier_wait... " >&6; }
+if ${ac_cv_search_pthread_barrier_wait+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  ac_func_search_save_LIBS=$LIBS
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+/* Override any GCC internal prototype to avoid an error.
+   Use char because int might match the return type of a GCC
+   builtin and then its argument prototype would still apply.  */
+#ifdef __cplusplus
+extern "C"
+#endif
+char pthread_barrier_wait ();
+int
+main ()
+{
+return pthread_barrier_wait ();
+  ;
+  return 0;
+}
+_ACEOF
+for ac_lib in '' pthread; do
+  if test -z "$ac_lib"; then
+    ac_res="none required"
+  else
+    ac_res=-l$ac_lib
+    LIBS="-l$ac_lib  $ac_func_search_save_LIBS"
+  fi
+  if ac_fn_c_try_link "$LINENO"; then :
+  ac_cv_search_pthread_barrier_wait=$ac_res
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext
+  if ${ac_cv_search_pthread_barrier_wait+:} false; then :
+  break
+fi
+done
+if ${ac_cv_search_pthread_barrier_wait+:} false; then :
+
+else
+  ac_cv_search_pthread_barrier_wait=no
+fi
+rm conftest.$ac_ext
+LIBS=$ac_func_search_save_LIBS
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_search_pthread_barrier_wait" >&5
+$as_echo "$ac_cv_search_pthread_barrier_wait" >&6; }
+ac_res=$ac_cv_search_pthread_barrier_wait
+if test "$ac_res" != no; then :
+  test "$ac_res" = "none required" || LIBS="$ac_res $LIBS"
+
+fi
+
+fi
+
 if test "$with_readline" = yes; then
 
 
@@ -15939,19 +15942,6 @@ esac
 
 fi
 
-ac_fn_c_check_func "$LINENO" "pthread_barrier_wait" "ac_cv_func_pthread_barrier_wait"
-if test "x$ac_cv_func_pthread_barrier_wait" = xyes; then :
-  $as_echo "#define HAVE_PTHREAD_BARRIER_WAIT 1" >>confdefs.h
-
-else
-  case " $LIBOBJS " in
-  *" pthread_barrier_wait.$ac_objext "* ) ;;
-  *) LIBOBJS="$LIBOBJS pthread_barrier_wait.$ac_objext"
- ;;
-esac
-
-fi
-
 ac_fn_c_check_func "$LINENO" "pwrite" "ac_cv_func_pwrite"
 if test "x$ac_cv_func_pwrite" = xyes; then :
   $as_echo "#define HAVE_PWRITE 1" >>confdefs.h
@@ -16058,6 +16048,23 @@ fi
 
 
 
+if test "$enable_thread_safety" = yes; then
+  ac_fn_c_check_func "$LINENO" "pthread_barrier_wait" "ac_cv_func_pthread_barrier_wait"
+if test "x$ac_cv_func_pthread_barrier_wait" = xyes; then :
+  $as_echo "#define HAVE_PTHREAD_BARRIER_WAIT 1" >>confdefs.h
+
+else
+  case " $LIBOBJS " in
+  *" pthread_barrier_wait.$ac_objext "* ) ;;
+  *) LIBOBJS="$LIBOBJS pthread_barrier_wait.$ac_objext"
+ ;;
+esac
+
+fi
+
+
+fi
+
 if test "$PORTNAME" = "win32" -o "$PORTNAME" = "cygwin"; then
 	# Cygwin and (apparently, based on test results) Mingw both
 	# have a broken strtof(), so substitute the same replacement

configure.ac

@@ -1143,7 +1143,6 @@ AC_SEARCH_LIBS(getopt_long, [getopt gnugetopt])
 AC_SEARCH_LIBS(shm_open, rt)
 AC_SEARCH_LIBS(shm_unlink, rt)
 AC_SEARCH_LIBS(clock_gettime, [rt posix4])
-AC_SEARCH_LIBS(pthread_barrier_wait, pthread)
 # Solaris:
 AC_SEARCH_LIBS(fdatasync, [rt posix4])
 # Required for thread_test.c on Solaris
@@ -1156,6 +1155,10 @@ AC_SEARCH_LIBS(shmget, cygipc)
 # *BSD:
 AC_SEARCH_LIBS(backtrace_symbols, execinfo)
 
+if test "$enable_thread_safety" = yes; then
+  AC_SEARCH_LIBS(pthread_barrier_wait, pthread)
+fi
+
 if test "$with_readline" = yes; then
   PGAC_CHECK_READLINE
   if test x"$pgac_cv_check_readline" = x"no"; then
@@ -1744,7 +1747,6 @@ AC_REPLACE_FUNCS(m4_normalize([
 	mkdtemp
 	pread
 	preadv
-	pthread_barrier_wait
 	pwrite
 	pwritev
 	random
@@ -1755,6 +1757,10 @@ AC_REPLACE_FUNCS(m4_normalize([
 	strtof
 ]))
 
+if test "$enable_thread_safety" = yes; then
+  AC_REPLACE_FUNCS(pthread_barrier_wait)
+fi
+
 if test "$PORTNAME" = "win32" -o "$PORTNAME" = "cygwin"; then
 	# Cygwin and (apparently, based on test results) Mingw both
 	# have a broken strtof(), so substitute the same replacement