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

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