pgsql: Suppress clang's unhelpful gripes about -pthread switch being un

Suppress clang's unhelpful gripes about -pthread switch being unused.

Considering the number of cases in which "unused" command line arguments
are silently ignored by compilers, it's fairly astonishing that anybody
thought this warning was useful; it's certainly nothing but an annoyance
when building Postgres.  One such case is that neither gcc nor clang
complain about unrecognized -Wno-foo switches, making it more difficult
to figure out whether the switch does anything than one could wish.

Back-patch to 9.3, which is as far back as the patch applies conveniently
(we'd have to back-patch PGAC_PROG_CC_VAR_OPT to go further, and it doesn't
seem worth that).

Branch
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/6347bdb31448f812cd726cfb3cdcdecf41a38b19

Modified Files
--------------
configure    |   66 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
configure.in |    7 +++++++
2 files changed, 73 insertions(+)