1.0.37.47: less pain for building threads on Darwin

1.0.37.47: less pain for building threads on Darwin

 * Use RUN-PROGRAM for impure tests everywhere. Not only is it better
   to use the more-portable solution everywhere, we had a huge number
   of bogus failures on thread tests on Darwin due to interactions
   between fork() and thread stack cleanup.

   Addresses Launchpad bug #310208.

 * Make tests depending on mutex timeout punt on lutex platform, and
   make several test which are prone hang or crash into LDB punt on
   Darwin. ("Punt" here means "call ERROR" so we get a test failure.)

 * Disable mailbox tests prone to hang on Darwin.

 ...so building threads on Darwin means one actually has a prayer or
 running the tests with useful results -- and the failures are real
 Darwin problems.