Mac OS X: Fix crash in Harfbuzz-NG

CoreText does not support multithreading, so we would get a
crash in tst_qglthreads. So we disable multithreading for
HB-NG.

Task-number: QTBUG-38762
Change-Id: I0473037c16017ff77cbba9b11fc0b396775ef789
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>