pgsql: 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(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/2568537.1615603606%40sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/de91c3b976cfacddacd45a9b52046264c0e44b11

Modified Files
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configure | 145 +++++++++++++++++++++++++++++++----------------------------
configure.ac | 10 ++++-
2 files changed, 84 insertions(+), 71 deletions(-)