At least on my macOS 10.12.1, PTHREAD_STACK_MIN is 8192 (so stacksize was
substantially smaller than 12MB), and without this change there were randomly
looking failures in e.g. JunitTest_chart2_unoapi with a -fsanitize=address
build.

Change-Id: Icfe989a0e5097a9a0ae76c6e0f6ffcca18271245