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.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